Specimens of Triarthrus retaining appendages were first obtained by Mr. W. S. Valiant from the dark carbonaceous Utica shale near Rome, New York, in 1884, but no considerable amount of material was found until 1892. The first specimens were sent to Columbia University, and were described by Doctor W. D. Matthew (1893). This article was accompanied by a plate of sketches, showing for the first time the presence of antennules in trilobites and indicating something of the endopodites and exopodites of the appendages of the cephalon, thorax, and pygidium. Specimens had not yet been cleaned from the lower side, so that no great amount could then be learned of the detailed structure. Matthew concluded that "The homology with Limulus seems not to be as close in Triarthrus as in the forms studied by Mr. Walcott; but the characters seem to be of a more comprehensive type, approaching the general structure of the other Crustacea rather than any special form."

Professor Beecher's first paper, dated October 9, 1893, merely mentioned the fact that the Yale University Museum had obtained material from Valiant's locality, but was quickly followed by a paper read before the National Academy of Sciences on November 8, and published in December, 1893. This paper described particularly the thoracic appendages.

This was followed in January (1894 A) by an article in which some information about the mode of occurrence of the specimens was added, and in April (1894 B), the limbs of the pygidium were described and figured. The determination of the structure of the appendages of the head evidently presented some difficulty, for the article describing this portion of the animal did not appear until the next February (1895 A). This cleared up the ventral anatomy of Triarthrus, and was followed by a short article (1896 A) accompanied by a restoration of the trilobite showing all the appendages.

This ended Professor Beecher's publications on Triarthrus until his final paper in 1902, although he contributed some of his results and figures to his chapter on the trilobites in the Eastman-Zittel Text-book of Paleontology in 1900.

The discovery of these excellent specimens had of course excited very great interest. Doctor Walcott also studied a number of specimens from Valiant's locality, and published in 1894, with some original figures, the results of his comparison of the appendages of Triarthrus with those of Calymene and Ceraurus.

In his article on the "Systematic Position of the Trilobites," Bernard (1894) used the results of Professor Beecher's studies of 1893, and also quoted the papers by Matthew (1893) and Walcott (1894), though the article by the latter appeared too late to be used except for a note added while Bernard's paper was in press. A final footnote quoted from Professor Beecher's paper of April, 1894 (1894 B).

Œhlert (1896) gave an excellent summary in French of the work of Beecher and Walcott on Triarthrus, with reproductions of many of their figures.

Valiant (1901) in a non-technical article described his long search for trilobites with antennas. The discovery of the wonderful pyritized trilobites at Cleveland's Glen near Rome was not the result of a lucky accident, but the culmination of eight years of labor in a locality especially selected on account of the fineness of grain of the shale.

After 1896, Professor Beecher turned his attention largely to the problem of the classification of trilobites, and while he continued the arduous task of cleaning the matrix from specimens of Triarthrus and Cryptolithus he did not again publish upon the subject of appendages until forced to do so by the doubts cast by Jaekel (1901) upon the validity of his earlier conclusions. Because of certain structures which he thought he had interpreted correctly from a poorly preserved specimen of Ptychoparia, Jaekel came to the conclusion that Beecher's material was not well preserved. Professor Beecher would have taken much more kindly to aspersions upon his opinions than to any slight upon his beloved trilobites, and his article on the "Ventral Integument of Trilobites" of 1902 was designed not only as an answer to Jaekel, but also to show by means of photographs the unusually perfect state of preservation of the specimens of Triarthrus. This article, like so many describing the appendages of trilobites, beginning with Matthew's, was published in two places (Beecher 1902).

Most of Beecher's papers, except the last one, were reprinted in the volume entitled "Studies in Evolution," published by Charles Scribner's Sons at the time of the Yale Bicentennial in 1901. The part pertaining particularly to Triarthrus is on pages 197 to 219.

Moberg (1907), in connection with a specimen of Eurycare angustatum which he thought preserved some appendages, described and illustrated some of the appendages of Triarthrus.

The most recent discussion of Triarthrus, with some new figures, is by Walcott (1918, p. 135, pls. 29, 30). He gives a summary of Beecher's work with numerous quotations. The principal original contribution is a discussion of the form and shape of the appendages before they were flattened out in the shale. He found also what he thought might possibly be the remains of epipodites on three specimens, one of which he illustrated with a photograph. I have seen nothing which could be interpreted as such an organ in the many specimens I have studied.

A point in which Walcott differs from Beecher in the interpretation of specimens is in regard to the development of the endopodites of small pygidia. Beecher (1894 B, pl. 7, fig. 3) illustrated a series of endopodites which he likened to the endites of a thoracic limb of Apus. Doctor Walcott finds that specimens in the United States National Museum show slender endopodites all the way to the back of the pygidium, and thinks that Beecher mistook a mass of terminal segments of exopodites for a series of endopodites. On careful examination, however, the specimen shows, as Beecher indicated, a series of endopodites in undisturbed condition (No. 222, our pl. 4, fig. 5).

Restoration of Triarthrus.

One of the more important points noted in the later studies of Triarthrus is that the gnathites of the cephalic appendages are much less like the endobases under the thorax than Beecher earlier thought, and showed in his restored figures and in his model. The four gnathites of each side are curved, flattened, not club-shaped, and so wide and so close together that they overlap one another. The metastoma is somewhat larger and more nearly circular than Beecher's earlier preparations led him to suppose.

The restoration here presented is modified only slightly from the one designed by Professor Beecher, and the modifications are taken principally from figures published by him. The gnathites are drawn in form more like that shown by the specimens and his figures in the American Geologist (1895 A), and the metastoma is taken from one of the specimens. On the thorax the chief modification is in the addition of a considerable number of spines to the endopodites. In spite of the trivial character of most of these changes, they emphasize one of the important characteristics of Triarthrus the regional differentiation of the appendages.

It should be pointed out that although Triarthrus is usually considered to be a very primitive trilobite, its appendages are more specialized than those of any of the others known. This is shown in their great length, the double curvature of the antennules, the differentiation of four pairs of endobases on the cephalon as gnathites, and the flattening of the segments of the posterior endopodites. These departures from the uniformity existing among the appendages of the other genera lead one to question whether the genus is really so primitive as has been supposed.

Relation of the Cephalic Appendages to the Markings on the Dorsal Surface of the Glabella.

Triarthrus becki is usually represented as having four pairs of glabellar furrows, but the two pairs at the front are exceedingly faint and the first of them is hardly ever visible, though that it does exist is proved by a number of authentic specimens. The neck furrow is narrow and sharply impressed, continuing across the glabella with a slightly backward curvature. In front of it are two pairs of linear, deeply impressed furrows which in their inward and backward sweep are bowed slightly forward, the ends of the corresponding furrows on opposite sides nearly meeting along the crest of the glabella. In front of these, near the median line, is a pair of slight indentations, having the appearance and position of the inner ends of a pair of furrows similar to those situated just behind them.

In front of and just outside this pair are the exceedingly faint impressions of the anterior pair of furrows, these, as said above, being but seldom seen. They are short, slightly indented linear furrows which have their axes perpendicular to the axis of the cephalon, and do not connect with each other or with the dorsal furrows. The latter are narrow, sharply impressed, and merge into a circumglabellar furrow at the front. In front of the circumglabellar furrow is a very narrow rounded ridge, but the anterior end of the glabella is very close to the margin of the cephalon.

Specimen No. 214, which was cleaned from the dorsal side, shows the posterior tip of the hypostoma, apparently in its natural position, 3.5 mm. back from the anterior margin. The entire length of the cephalon is 6 mm., so that the hypostoma reaches back slightly over one half the length (0.583). The greater part of it has been cleaned off, and one sees the proximal portions of the antennules, which are apparently attached just at the sides of the hypostoma, 2.5 mm. apart and 2.25 mm. back from the anterior edge of the cephalon. This position is distinctly within the outline of the glabella and corresponds approximately to the location of the second pair of glabellar furrows. Specimens 214, 215, 216, 217, and 219 all seem to show the same location for the bases of the antennules. Specimen 220 is the one in which the basal shafts are best preserved and the points of attachment seem to be further apart in it than in any of the others. This specimen is 38 mm. long, and the bases of the antennules are 5.5 mm. apart and 4 mm. behind the anterior margin. As the specimen is cleaned from the ventral side, the dorsal furrows do not show distinctly, but another specimen of about the same size (No. 228, 38.5 mm. long) has the dorsal furrows 8 mm. apart 4 mm. back of the anterior margin.

On the same slab with specimens 209 and 210 there is an individual which, although retaining the test, has had the proximal ends of the antennules so pressed against it that the course of the one on the left side is readily visible. It originates in a small oval mound whose posterior margin impinges upon the third glabellar furrow near the middle of its course, and just outside the outer end of the second glabellar furrow. The cephalon of this specimen is 5 mm. long, and the point of origin of the left antennule is 2.75 mm. in front of the posterior margin and 0.75 mm. from the dorsal furrow.

It is therefore evident that the antennules in this species are not attached beneath the dorsal furrows, but within them and opposite the second pair of glabellar furrows.

All cephalic appendages behind the antennules are attached somewhat within the dorsal furrows, the first pair as far forward as the antennules and the last pair apparently under the anterior edge of the neck ring. They do not appear to correspond in position to the posterior glabellar furrows and neck ring, being more crowded. The last pair is attached to appendifers beneath the nuchal segment, and the first pair beneath the third glabellar furrows. There are no depressions on the dorsal surface corresponding to the points of attachment of the mandibles.

Anal Plate.

Professor Beecher, during his first studies of Triarthrus, found no appendages pertaining to the anal segment, but later evidently came upon a spinose anal plate which he caused to be figured. The specimen (No. 201) on which this appendage is preserved is cleaned from the dorsal side, and the anal plate is a small, bilaterally symmetrical, nearly semicircular structure margined with small spines. Specimen 202 also shows the same plate (pl. 5, fig. 6), but it is imperfectly preserved. It has a large perforation in the anterior half. Both of these specimens are in the Yale University Museum.

Fig. 11.—Triarthrus becki Green. Anal plate of specimen 65525 in the U. S. National Museum. Drawn by Doctor Wood. × 20.