Agassiz, L.

1873.—Discovery of the basal joint of legs of trilobites. Amer. Nat., vol. 7, pp. 741-742.

Angelina N. P.

1854.—PalÆontologia Scandinavica, pars 1, Crustacea formationis transitionis.

Audouin, J. V.

1821.—Recherches sur les rapports naturels qui existent entre les trilobites et les animaux articulÉs. Ann. Gen. Sci. Phys. Nat. Bruxelles, vol. 8, p. 233, pl. 26. 1822. Isis (Encycl. Zeitung), Oken., vol. 10, p. 87, pl. 1, No. 4, figs. 1-5.

Barrande, J.

1852.—SystÊme Silurien du centre de la BohÊme, vol. 1, pp. 226-230, and 629, pl. 30, figs. 38, 39.

1872.—Ibid., vol. 1, Suppl., p. 180, pl. 4.

Barth, Hermann von.

1875.—Die Stellung der Trilobiten in zoologischen Systeme. Das Ausland, 26. Jahrg., p. 2 5.

Beecher, C. E.

1893.—On the thoracic legs of Triarthrus. Amer. Jour. Sci. (3), vol. 46, pp. 367-370, 467-470, text figs. 1-3.

1894 A.—On the mode of occurrence, and the structure and development of Triarthrus becki. Amer. Geol., vol. 13, pp. 38-43, pl. 3.

1894 B.—The appendages of the pygidium of Triarthrus. Amer. Jour. Sci. (3), vol. 47, pp. 298-300, pl. 7, text fig, 1.

1895 A.—Further observations on the ventral structure of Triarthrus. Amer. Geol., vol. 15, pp. 91-100, pls. 4-5.

1895 B.—Structure and appendages of Trinucleus. Amer. Jour. Sci. (3), vol. 49, pp. 307-311, pl. 3.

1895 C.—The larval stages of trilobites. Amer. Geol., vol. 16, pp. 166-197, pls. 8-10.

1896 A.—The morphology of Triarthrus. Amer. Jour. Sci. (4), vol. 1, pp. 251-256, pl. 8; Geol. Mag., dec. 4, vol. 3, pp. 193-197, pl. 9.

1896 B.—On a supposed discovery of the antennas of trilobites by LinnÆus in 1759. Amer. Geol., vol. 17, pp. 303-306, text figs. 1-3.

1897 A.—Outline of a natural classification of trilobites. Amer. Jour. Sci. (4), vol. 3, pp. 89-106, 181-207, pl. 3.

1897 B.—Remarks on Kingsley's "Systematic position of the trilobites." Amer. Geol., vol. 20, pp. 38-40.

1900.—Trilobita. Eastman-Zittel Text-book of Paleontology, vol. 1, pp. 607-638, text figs. 1261-1331; ed. 2, 1913, p. 700. London.

1901.—Structure and development of trilobites. In "Studies in Evolution," pp. 109-225. New York and London.

1902.—The ventral integument of trilobites. Amer. Jour. Sci. (4), vol. 13, pp. 165-174, pls. 2-5, text fig. 1; Geol. Mag., dec. 4, vol. 9, pp. 152-162, pls. 9-11, text figs. 1-8.

Bernard, H. M.

1892.—The ApodidÆ.

1893.—Trilobites with antennÆ at last! Nature, vol. 48, p. 582.

1894.—The systematic position of the trilobites. Quart. Jour. Geol. Soc., London, vol. 50, pp. 411-434, text figs. 1-17.

1895 A.—The zoological position of the trilobites. Science Prog., vol. 4, pp. 33-49.

1895 B.—Supplementary notes on the systematic position of the trilobites. Quart. Jour. Geol. Soc., London, vol. 51, pp. 352-360, figs. A-C.

Beyrich, E.

1846.—Untersuchungen ueber Trilobiten. 2. StÜck, p. 30, pl. 4, fig. 1c.

Billings, E.

1870.—Notes on some specimens of Lower Silurian trilobites. Quart. Jour. Geol. Soc., London, vol. 26, pp. 479-486, pls. 31-32. Abstract in Geol. Mag., vol. 7, p 291, and Nature, vol. 2, p. 94.

Brongniart, A.

1822.—Histoire naturelle des crustacÉs fossiles. Paris.

BrÜnnich, F. E.

1781.—Beskrivelse over trilobiten, en dyreslaegt og dens arter, med en ney arts aftegning. Nye Samlig of det Kong. Danske Vidensk. Selskabs. Skriften, Copenhagen.

Burling, L. D.

1916.—PÆdeumias and the MesonacidÆ, with description of a new species, having at least 44 segments, from the Lower Cambrian of British Columbia. Ottawa Nat., vol. 30, pp. 53-58, pl. 1.

1917.—Was the lower Cambrian trilobite supreme? Ibid., vol. 31, pp. 77-79, text figs. 1-2.

Burmeister, H.

1843.—Die Organisation der Trilobiten. Berlin.

1846.—The organization of trilobites, deduced from their living affinities. Eng. translation, Ray Society, London.

Calman, W. T.

1909.—Crustacea, in "A treatise on zoology," edited by Sir Ray Lankester. London.

1919. Dr. C. D. Walcott's researches on the appendages of trilobites. Geol. Mag., dec. 6, vol. 6, pp. 359-363, pl. 8, text fig. 1.

Carpenter, G. H.

1903.—On the relationships between the classes of Arthropoda. Proc. Roy. Irish Acad., vol. 24, pp. 320-360, pl. 6.

Castelnau, F. DE.

1843.—Systeme Silurien de l'AmÉrique Septentrionale, p. 15, pl. 2, figs. 1, 4.

Clarke, J. M.

1888.—The structure and development of the visual area in the trilobite, Phacops rana Green. Jour. Morph., vol. 2, pp. 253-270, pl. 1.

Crampton, G. C.

1916.—The phylogenetic origin and the nature of the wings of insects, according to the paranotal theory. Jour. New York Entomol. Soc., vol. 24, pp. 1-39, pls. 1, 2.

1919.—The evolution of the arthropods and their relatives, with especial reference to insects. Amer. Nat, vol. 53, pp. 143-179.

Dalman, J. W.

1826.—Om PalÆaderna eller de sÅ kallade Trilobiterna. Stockholm, Acad. Handl., pp. 113-152, 226-294.

1828.—Ueber die PalÆaden, oder die sogennanten Trilobiten. Nuremberg.

Dana, J. D.

1871.—On the supposed legs of the trilobite, Asaphus platycephalus. Amer. Jour. Sci. (3), vol. 1, pp. 320-321, 386; Ibid. (3), vol. 3, 1872, pp. 221-222. Also printed in Ann. Mag. Nat. Hist, vol. 7, 1871, pp. 366, 451.

Dekay, J. E.

1824.—Observations on the structure of trilobites, and description of an apparently new genus. Ann. Lye. Nat. Hist. New York, vol. I, p. 174, 2 pls.; Isis (Encycl. Zeit.), Oken, 1825 and 1832.

Dollo, L.

1910.—La palÉontologie Éthologique. Bull. Soc. Beige de Geol., Pal., et d'Hydrol., vol. 23, pp. 377-421, figs. 1-13, pls. 7-11.

Eichwald, E. VON.

1825.—Geognostico-zoologicÆ per Ingriam Marisque Baltici Provincias nee non de trilobitis observationes. Section 45.

1858.—BeitrÄge zur geographischen Verbreitung der fossilen Thiere Russlands. Bull. Soc. Imp. des Natural, de Moscou, vol. 30, 1855-1857, p. 204.

1860.—LethÆa Rossica, pl. 21.

1863.—BeitrÄge zur nÄhern Kenntniss der in meiner LethÆa Rossica beschriebenen IlÆnen. Bull. Soc. Imp. des Natural, de Moscou, vol. 36, p. 408.

Emmrich, H. F.

1839.—De trilobitis dissertatio petrefactologica, etc. Berlin.

Exner, S.

1891.—Die Physiologic der facettirten Augen von Krebsen und Insecten. Leipzig and Vienna. Pp. 33-35, pl. 2, figs. 18-19.

Finch, G. E.

1904.—Notes on the position of the individuals in a group of Nileus vigilans found at Elgin, Iowa. Proc. Iowa Acad. Sci. for 1903, vol. 11, pp. 179-181, pl. 14.

Gegenbaur, C.

1878.—Elements of comparative anatomy. Eng. ed. (Bell and Lankester). London.

Goldfuss, A.

1828.—Observation sur le place qu'occupent les trilobites dans le rÈgne animal. Ann. Sci. Nat., Zoologie, vol. 15, p. 83, pl. 2, figs. 5, 7, 9, 10.

Green, J.

1839 A.—The inferior surface of the trilobite discovered. The Friend, Philadelphia, March 16.

1839 B.—The inferior surface of the trilobite discovered. Illustrated, with colored models. Philadelphia.

1839 C.—Remarks on the trilobites. Amer. Jour. Sci. (1), vol. 37, p. 25 et seq.

1840.—An additional fact, illustrating the inferior surface of Calymene bufo. Ibid., vol. 38, p. 410.

Handlirsch, A.

1906.—Ueber Phylogenie der Arthropoden. Verhandl. d. k. k. zool.-bot. Gesell., Vienna, Jahrg. 1906, pp. 88-103.

1907.—Functionswechsel einiger Organe bei Arthropoden. Ibid., Jahrg. 1907, pp. 153-158.

1908.—Die fossilen Insekten. Leipzig.

1914.—Eine interessante Crustaceenform aus der Trias der Vogesen. Verhandl. d. k. k. zool.-bot. Gesell., Vienna, Jahrg. 1914, pp. 1-7, pls. 1, 2.

Hawle, I., and Corda, A. J. C.

1847.—Prodrom einer Monographie der boehmischen Trilobiten, pp. 9, 24, 56, pl. 2, fig. 10; pl. 3, fig. 15; pl. 4, fig. 33b-g.

Jaekel, O.

1901.—BeitrÄge zur Beurtheilung der Trilobiten, Theil I. Zeits. d. deutsch. geol. Gesell., Bd. 53, pp. 133-171. Pis. 4-6, text figs. 1-30.

Kingsley, J. S.

1897.—The systematic position of the trilobites. Amer. Geol., vol. 20, pp. 33-38.

Koenen, A. von.

1872.—Ueber die Organisation der Trilobiten. Verhandl. d. naturhist. Ver. d. preuss. Rheinl. u. Westphalen, vol. 29, C, pp. 93-95.

1880.—Ueber die Unterseite der Trilobiten. Neues Jahrb. f. Min.., Geol., u. Pal,, Bd. 1, pp. 430-432. pl. 8.

Lang, A.

1891.—Text-book of comparative anatomy, Eng. ed. (Bernard). London.

Lankester, E. R.

1881.—Observations and reflections on the appendages and on the nervous system of Apus cancriformis. Quart. Jour. Micros. Soc., vol. 21, pp. 343-376.

Laurie, M.

1911.—A reconstructed trilobite. Nature, vol. 88, p. 26.

Lindstroem, G.

1901.—Researches on the visual organs of the trilobites. K. svenska Vet.-Akad. Handl., new ser., vol. 34, pp. 1-86, pls. 1-6.

LinnÉ, K.

1759.—Petrificatet entomolithus paradoxus sÅdant, som det finnes uti Hans Excellence Riks. RÅdets HÖgoÄlborne Herr Grefve C. G. Tessins Samling. K. svenska Vet.-Akad. Handl., vol. 20, pp. 21, 22, pl. 1, fig. 1.

Matthew, W. D.

1893.—On antennÆ and other appendages of Triarthrus becki. Amer. Jour. Sci. (3), vol. 46, pp. 121-125, pl. 1; Trans. New York Acad. Sci., vol. 12, pp. 237-241, pl. a.

McCoy, F.

1846.—A synopsis of the Silurian fossils of Ireland, p. 42.

Mickleborough, J.

1883.—Locomotory appendages of trilobites. Jour. Cincinnati Soc. Nat. Hist., vol. 6, pp. 200-204; Geol. Mag., dec. 3, vol. 1, 1884, pp. 80-84; Amer. Jour. Sci. (3), vol. 27, 1884, p. 409. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal., Bd. 1, 1885, p. 477.

Miller, S. A.

1880.—Silurian ichnolites, with definitions of new genera and species. Jour. Cincinnati Soc. Nat. Hist, vol. 2, pp. 217-218, fig.

Milne-Edwards, H.

1881.—Compte rendu des nouvelles recherches de M. Walcott relatives À la structure des trilobites, suivi de quelques considÉrations sur l'interprÉtation des faits ainsi constatÉs. Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, pp. 1-33, pls. 10-12. Paris.

Moberg, J. C.

1902.—Bidrag till KÄnnedomen om trilobiternas byggnad. Geol. FÖren FÖrhandl., Bd. 24, pp. 295-302; pl. 3, text fig. 1.

1907.—Om ett gÄtfultt fossil frÄn sveriges olenidskiffer samt en kort ofversigt af viktigase data rorande trilobiternas ventrala skelettdelar. Ibid., Bd. 29, Heft 5, pp. 265-272, pl. 4, fig. 2; pl. 5, fig. 1.

Œhlert, D. P.

1896.—RÉsumÉ des derniers travaux sur l'organisation et le developpement des trilobites. Bull. Soc. GÉol. France, ser. 3, vol. 24, pp. 97-116, text figs. 1-34.

Packard, A. H.

1872.—On the development of Limulus polyphemus. Mem. Boston Soc. Nat. Hist., vol. 2, pp. 155-202, pls. 3-5.

1880.—The structure of the eye of trilobites. Amer. Nat., vol. 14, pp. 503-508.

1882.—On the homologies of the crustacean limb. Ibid., vol. 16, pp. 785-799, figs. 11, 12.

Pander, C.

1830.—BeitrÄge zur Geognosie des russischen Reiches. St. Petersburg.

Peach, B. N.

1882.—On some fossil myriopods from the Lower Old Red Sandstone of Forfarshire. Proc. Roy. Physical Soc., Edinburgh, vol. 7, pp. 177-187, pl. 2.

1899.—O some new myriopods from the PalÆozoic rocks of Scotland. Ibid., vol. 14, pp. 113-126, pl. 4.

Quenstedt, A.

1837.—Beitrag zur Kenntniss der Trilobiten, mit besonderer RÜcksicht auf ihre bestimmte Gliederzahl. Archiv f. Naturg., Berlin, 3. Jahrg., 1 Bd., pp. 337-352.

Raymond, P. E.

1910.—On two new trilobites from the Chazy near Ottawa, Ontario. Ottawa Nat., vol. 24, pp. 129-134, pl. 2.

1917.—Beecher's classification of trilobites, after twenty years. Amer. Jour. Sci. (4), vol. 43, pp. 196-210, text figs. 1-3.

Raymond, P. E., and Barton, D. C.

1913.—A revision of the American species of Ceraurus. Bull. Mus. Comp. Zool., vol. 54, pp. 525-543. pls. 1, 2, 3 text figs. 1-3.

Reed, F. R. C.

1916.—The genus Trinucleus. Pt. 4. Geol. Mag., dec. 6, vol. 3, pp. 121, 122.

Richter, R.

1848.—Bitrag zur PalÆeontologie des ThÜringer Waldes. Dresden and Leipzig.

Ringueberg, E. N. S.

1886.—A trilobite track illustrating one mode of progression of the trilobites. Proc. Amer. Assoc. Adv. Sci., vol. 35, p. 228 (abstract only).

Ruedemann, R.

1916.—The presence of a median eye in trilobites. Bull. New York State Mus., No. 189. pp. 127-143, pls. 34-36.

Schlotheim, E. F. von.

1823.—NachtrÄge zur Petrefactenkunde, II. Gotha.

Six, Achille.

1884.—Les appendices des trilobites d'aprÈs M. Ch. D. Walcott. Ann. Soc. Geol. du Nord, vol. 11, pp. 228-236.

Spencer, W. K.

1903.—The hypostomic eyes of trilobites. Geol. Mag., dec. 4, vol. 10, pp. 489-492.

Staff, Hans v., and Reck, Hans.

1911.—Ueber die Lebensweise der Trilobiten. Eine entwicklungsmechanische Studie. Gesell. naturforsch. Freunde, Sitzb., pp. 130-146, figs. 1-20.

Sternberg, K. M.

1830.—Ueber die Gliederung und die FÜsse der Trilobiten. Isis (Encycl. Zeitung), Oken, p. 516, pl. 5, figs. 1-3.

Stokes, C.

1823.—On a trilobite from Lake Huron. Trans. Geol. Soc., London, ser. 2, vol. 1, p. 208, pl. 27.

Swinnerton, H. H.

1919.—The facial suture of the trilobite. Geol. Mag., dec. 6, vol. 6, pp. 103-110.

TÖrnquist, S. L.

1896 A.—On the appendages of trilobites. Ibid., dec. 4, vol. 3, p. 142.

1896 B.—LinnÆus on the appendages of trilobites. Ibid., pp. 567-569.

Tothill, J. D.

1916.—The ancestry of insects, with particular reference to chilopods and trilobites. Amer. Jour. Sci. (4), vol. 42, pp. 373-383. text figs. 1-8.

Troedsson, G. T.

1918.—Om skanes Brachiopodskiffer. Lunds Universitets Arsskrift, n. f., Avd. 2, Bd. 15, Nr. 3. pp. 57-67, pl. 1, figs. 19-24.

Valiant, W. S.

1901.—Appendaged trilobites. The Mineral Collector, vol. 8, No. 7, pp. 105-112.

Volborth, A. von.

1858.—Ueber die Bewegungs-Organe der Trilobiten. Verhandl. russ. k. mineral. Gesell. zu St Petersburg, 1857-1858, p. 168.

1863.—Ueber die mit glatten Rumpfgliedern versehenen russischen Trilobiten, nebst einem Anhange ueber die Bewegungs-organe und ueber das Herz derselben. Mem. Acad. Imp. Sci. St. Petersburg, ser. 7, vol. 6, No. 2, pp. 44-47, pl. 1, fig. 12.

1866.—Ueber Herrn von Eichwald's Beitrag zu nÄheren Kenntniss der IllÆnen. Bull. Soc. Imp. des Natural, de Moscou, vol. 39, p. 40.

Wahlenberg, G.

1821.—Petrificata telluris Suecana examinata a Georgio Wahlenberg. Nova Acta Reg. Soc. Scient. Upsala, vol. 8.

Walcott, C. D.

1875.—Description of the interior surface of the dorsal shell of Ceraurus pleurexanthemus Green. Ann. Lye. Nat. Hist. New York, vol. II, pp. 159-162, pl. 11.

1876.—Preliminary notice of the discovery of the natatory and branchial appendages of trilobites. 28th Rept. New York State Mus. Nat. Hist., adv. sheets, pp. 89-92; published as full report in 1879.

1877.—Notes on some sections of trilobites. 31st Rept. New York State Mus. Nat. Hist., adv. sheets, pp. 61-63, pl. 1; published as full report in 1879. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal., Bd. 1, 1880, p. 428.

1879.—Notes upon the legs of trilobites. 31st Rept. New York State Mus. Nat. Hist., adv. sheets, p. 64.

1881.—The trilobite: New and old evidence relating to its organization. Bull. Mus. Comp. Zool., vol. 8, pp. 192-224, pls. 1-6.

1884.—The appendages of the trilobite. Science, vol. 3, pp. 276-279, figs. 1-3. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal.., Bd. 1, 1885, Referate, p. 102.

1894.—Note of some appendages of the trilobites. Proc. Biol. Soc. Washington, vol. 9, pp. 89-97, pl. 1; Geol. Mag., dec. 4, vol. 1, pp. 246-251, pl. 8.

1911.—Middle Cambrian Merostomata. Smithson. Misc. Coll., vol. 57, No. 2, pp. 17-40, pls. 2-7.

1912 A.—Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata. Ibid., No. 6, pp. 145-228, pls. 24-34, text figs. 8-10.

1912 B.—New York Potsdam-Hoyt fauna. Ibid., No. 9, pp. 251-304, pls. 37-49.

1913.—Eastman-Zittel Text-book of Paleontology, ed. 2, vol. 1, figs. 1343, 1376, 1377.

1916.—Ann Rept., Secretary Smithsonian Inst, for 1915, pl. 9.

1918.—Appendages of trilobites. Smithsonian Misc. Coll., vol. 67, No. 4, pp. 115-226, pls. 14-42.

Watase, S.

1890.—On the morphology of the compound eyes of arthropods. Johns Hopkins Univ., Studies from Biol. Lab., vol. 4, no. 6, p. 290 (footnote).

Woodward, H.

1870.—Note on the palpus and other appendages of Asaphus, from the Trenton limestone, in the British Museum. Quart. Jour. Geol. Soc., London, vol. 26, pp. 486-488, fig. 1. Abstract in Geol. Mag., dec. 1, vol. 7, p. 292, also in Nature, vol. 2, p. 94.

1871.—On the structure of trilobites. Geol. Mag., dec. 1, vol. 8, pp. 289-294, pl. 8.

1884.—Notes on the appendages of trilobites. Geol. Mag., dec. 3, vol. 1, pp. 162-165, 2 text figs.

1895.—Some points in the life history of the Crustacea in early PalÆozoic times. Quart. Jour. Geol. Soc., London, vol. 51, pp. lxx-lxxxviii, 1 pl.

---

[Pl 1]

PLATE 1.

Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. 1. Specimen 213. The dorsal test has been removed from the glabella, revealing the outline of the posterior end of the hypostoma, the proximal ends of the antennules, the gnathites, and incomplete endopodites of some appendages, × 5.43.

Fig. 2. Specimen 214. The head of a complete large specimen. Part of the thorax is shown on pl. 3, fig. 6. Note especially the form of the segments of the endopodites and of the anterior coxopodite on the right side, × 7.33.

Fig. 3. Specimen 217. This specimen shows better than any other the form of the gnathites of the cephalon. Note also the setÆ of the exopodites under the cheek at the right. The appearance of a hook on the posterior gnathite on the right may be accidental, but it does not show broken edges, × 6.85.

Fig. 4. Specimen 215. The ventral side of the cephalon of a small entire specimen. Shows well the form of some of the gnathites and a few of the endopodites. Note the unusual position of the antennules. × 7.63.

Fig. 5. Specimen 226. This specimen did not photograph well, but is important as showing the exopodites and endopodites emerging from under the cephalon. × about 6.

[171]

PLATE I.

[172]

[173]

---

[Pl 2]

PLATE 2.

Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. 1. Specimen 201. The entire specimen, details of which are shown in pl. 3, fig. 4 and pl. 4, figs. 1, 2. The dorsal test has been removed from the anterior segments on the right side. × 4.12.

Fig. 2. Specimen 206. A small individual with the endopodites, and the exopodites minus their setÆ; well preserved on the left side. Note the position of the antennules. The course of the facial suture is unusually well shown. × 10.

Fig. 3. Specimen 210. The specimen which served as the main basis for Professor Beecher's first figure of the appendages of the thorax, specimen 206 (fig. 2, this plate) having supplemented it. Note the "normal" position of the antennules and the extension of the appendages from beneath the pleural lobe. Specimens with the antennules in this position may possibly be males. × 4.

Fig. 4. Specimen 205. A small specimen with some of the appendages preserved, especially toward the posterior end, but particularly valuable for the unusually well preserved metastoma. × 11.

Fig. 5. Specimen 211. A small cephalon, cleaned from the ventral side, and showing well the gnathites which approach each other unusually closely on the median line. × 10.5.

Fig. 6. Specimen 219. An entire specimen of medium size, developed from the ventral side. It shows particularly well the "normal" curvature of the antennules, the change in form of the segments of the endopodites from cephalon to pygidium, and, along the axial lobe, the apodemes of the ventral integument. See also pl. 4, fig. 4. × 3.6.

[175]

PLATE II.

[176]

[177]

---

[Pl 3]

PLATE 3.

Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. 1. Specimen 204. See also text fig. 42 and pl. 4, fig. 6. The exopodites and endopodites of the first few segments of this specimen are better preserved than those of any other revealing them from the dorsal side, × 9.5.

Fig. 2. Specimen 220. A large individual exposed from the lower side. It shows well the endopodites and part of the exopodites, and, rather better than any other specimen, the endobases of the coxopodites. × 2.4.

Fig. 3. Specimen 216. A small entire specimen showing considerable of the detail of the appendages of the cephalon, and some of those of the remainder of the body, × 7.4.

Fig. 4. Specimen 201. This figure shows the details of the appendages of the left side and of the pygidium. Note the plate on the median line back of the pygidium, the sockets for spines, and the terminal spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4, figs. 1, 2. × 7.1.

Fig. 5. Specimen 207. One half of the posterior part of the thorax and pygidium, showing exopodites and endopodites as seen from the dorsal side, × 7.6.

Fig. 6. Specimen 214. The exopodites have been turned back nearly parallel to the axis of the shell. Notice particularly the long flattened setÆ and the spinose spatula-shaped terminal portion of each shaft. See also pl. 1, fig. 2. × 7.

[179]

PLATE III.

[180]

[181]

---

[Pl 4]

PLATE 4.

Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3, but showing more clearly some details of spines on the endopodites. × 12.66.

Fig. 2. Specimen 201. Three appendages on the right side of the thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. × 12.66.

Fig. 3. Specimen 223. A small crushed specimen which nevertheless shows well the appendages of the right side of the thorax, developed from the ventral side. Note coxopodites, exopodites, and endopodites, and that all appendages are moved equally laterally from their original position. × 11.4.

Fig. 4. Specimen 219. Another photograph, with different lighting, of the individual shown in pl. 2, fig. 6. This print brings out better the coxopodites and the folds of the ventral membrane. × 3.23.

Fig. 5. Specimen 222. This specimen is interesting, because it shows the endopodites in what is probably their natural position, that is, in a plane nearly vertical to the plane of the body, instead of being flattened down, as is usually the case. The appendages under the pygidium are unusually well preserved. × 12.

Fig. 6. Specimen 204. Photograph of the entire specimen of which a part is shown in text fig. 42 and pl. 3, fig. 1. × 4.5.

[183]

PLATE IV.

[186]

[185]

---

[Pl 5]

PLATE 5.

Photographs of Triarthrus becki, made by C. E. Beecher.

Fig. 1. Specimen 209. Photograph of the pygidium shown in pl. 6, fig. 2. This specimen shows especially well the way in which the exopodites of the pygidium decrease in length backward, × 11.5.

Fig. 2. Specimen 229. The under side of the posterior end of a medium-sized specimen, showing the appendages, especially the endopodites. On and among the limbs are scattered numerous minute spheres of pyrite, of the kind usually known as "trilobite eggs." They do not show very well in the photograph, but can be made out much more clearly with a hand lens, × 12.

Fig. 3. Specimen 230. A specimen showing the appendages of the posterior part of the thorax and the pygidium. The same individual is also shown in text fig. 44. Note particularly the form of the segments of the endopodites, and the spines on them, × 13.

Fig. 4. Specimen 227. The small doubly curved bodies shown in this figure lie under the axial portion of the cephalon and anterior part of the thorax. The specimen still has a very thin coating of matrix between it and the shell. Whether the curved bodies have anything to do with the trilobite is not known, × about 12.

Fig. 5. Specimen 221. A small individual which shows well the exopodites of the posterior part of the thorax. Note the spatulate terminations and the spines of the shaft, × 11.

Fig. 6. Specimen 202. Posterior part of the thorax and pygidium, showing endopodites and exopodites projecting under the dorsal test. Note the spiniferous plate on the median line, and the large opening in the anterior portion of it. × 9.75

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PLATE V.

[188]

[189]

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[Pl 60]

PLATE 6.

All figures except 4 and 5, from photographs by C. E. Beecher.

Fig. 1. Triarthrus becki. Specimen 203. A well preserved small individual, showing the appendages of the right side of the thorax. × 11.46.

Fig. 2. Triarthrus becki. Specimen 209. A well preserved individual, showing the antennules and some appendages of thorax and pygidium. For detail of the pygidium, see pl. 5, fig. 1. × 4.

Fig. 3. Triarthrus becki. Specimen 218. Ventral side of the pygidium and greater part of the thorax of an individual of medium size. Note especially the relation of exopodites to endopodites of the last two thoracic segments. A drawing of these appendages is shown on text fig. 43. × 4,3.

Figs. 4 and 5. Endopodites, probably from a species of Calymene. These specimens, with several others, are on a small slab of limestone from the Point Pleasant (Trenton) beds opposite Cincinnati, Ohio. Specimen in the U. S. National Museum. Photographs by R. S. Bassler.

Fig. 6. Acidaspis trentonensis Walcott. Both the specimen, No. 245, and the photograph are poor, but show that in this genus the endopodites are like those of Triarthrus. × 8.5.

Fig. 7. Cryptolithus tessellatus Green. Specimen 234. This specimen shows well the backward directed antennules and also the outer segments of some of the cephalic endopodites. × 11.

PLATE VI.

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PLATE 7.

Photographs of Cryptolithus tessellatus Green, made by C. E. Beecher.

Fig. 1. Specimen 233. The best preserved individual, the one from which Professor Beecher's drawing (text fig. 45) was made, and which served as the principal basis for the restoration (text fig. 20). Note the long, backward directed antennules, the abrupt backward turn of the outer portions of the endopodites, the way in which the exopodites extend beyond the endopodites, and the fact that alt are beneath the cover of the dorsal shield. The hypostoma is turned entirely around. × 10.9.

Fig. 2. Specimen 235. Half of the thorax and pygidium, with the appendages revealed from the ventral side. Note the abrupt manner in which the outer portions of the endopodites are turned backward. See also pl. 8, fig. 3, and pl. 9, fig. 1 (right half). × 14.45.

Fig. 3. Specimen 236. Detail from fig. 4, to show the blade-like setÆ of the exopodites and the numerous terminal spines of the endopodites. × 30.

Fig. 4. Specimen 236. The appendages of the thorax and pygidium, seen from the lower side. Specimen 236 is the right half of the same individual from which specimen 235 was obtained. Note the interarticular membranes between the segments of the endopodites and the blade-like setÆ of the exopodites. See also pl. 9, fig. 1 (left side). × 19.

Fig. 5. Specimen 236. The same specimen, seen from the dorsal side, showing, when the test is removed, the long blade-like setÆ of the exopodites. See also pl. 9, fig. 2 (right half). × 19.

PLATE VII.

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PLATE 8.

Photographs of Cryptolithus tessellatus Green, made by C. E. Beecher.

Fig. 1. Specimen 231. A nearly complete individual, cleaned from the ventral side and showing obscurely the hypostoma and fragments of numerous appendages. Note the lines of appendifers along the sides of the axial lobe. × 11.

Fig. 2. Specimen 232. Although this is not very well preserved, it shows more of the cephalic appendages than any other. Even so, only just enough is shown to indicate that they were similar to those on the thorax. × 12.

Fig. 3. Specimen 235. Dorsal side of the appendages of the thorax and pygidium. See pl. 7, fig. 2 for the ventral view. On pl. 9, fig. 2 (left side) is a drawing taken from the same specimen. × 11.

Fig. 4. Specimen 238. Part of a thorax and pygidium, seen from the ventral side. The series of heavy segments shown in the upper part do not belong to one appendage, but are the distal ends of several endopodites. See also text fig. 46 for a drawing of this specimen. × 18.

Fig. 5. Specimen 237. Pygidium and part of the thorax, with some of the appendages. × 11.

PLATE VIII.

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PLATE 9.

Cryptolithus tessellatus Green. Upper drawing by C. E. Beecher; lower drawing by Miss F. E. Isham, under the direction of C. E. Beecher.

Fig. 1. Appendages of the thorax and pygidium, seen from the ventral side. These are not restorations, but drawings from the halved individual numbered 236 (right side of drawing) and 235. For photographs of these specimens, see pl. 7, figs. 2, 4. × 20.

Fig. 2. Appendages of the thorax and pygidium, seen from the dorsal side. Same specimen as in fig. 1. For photographs, see pl. 7, fig. 5, and pl. 8, fig. 3. × 20.

PLATE IX.

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PLATE 10.

From photographs made by C. E. Beecher.

Fig. 1. Isotelus latus Raymond. Ventral surface of the specimen in the Victoria Memorial Museum at Ottawa, Canada. Note the large, club-shaped coxopodites and the more slender endopodites. The first large coxopodite back of the hypostoma belongs to the last pair of cephalic appendages. The coxopodite of the appendage in front of it is seen turning in beneath the tip of the hypostoma. × 2.

Fig. 2. Isotelus maximus Locke. The ventral side of the specimen described by Mickleborough and now in the U. S. National Museum. The tips of the hypostoma may be seen at the front, and the first two pairs of coxopodites behind them belong to the last two pairs of appendages of the cephalon. Note how much stronger the coxopodites are than the endopodites. The appendages of the pygidium show but poorly, × 1.45.

PLATE X.

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PLATE 11.

Ceraurus pleurexanthemus Green. A restoration of the ventral surface and appendages, made by Doctor Elvira Wood, under the supervision of the writer, from data obtained from the translucent slices prepared and described by Doctor Walcott. × 5.

PLATE XI.

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