Specimen No. 233 (pl. 7, fig. 1; text fig. 45).

This is the best preserved entire specimen. It is developed from the lower side, and shows the hypostoma, antennules, and a few fragmentary appendages of the cephalon, the outer portions of the exopodites of thorax and pygidium on both sides, and the endopodites on the left side.

The hypostoma is imperfectly preserved and is turned completely around, so that the anterior margin is directed backward, and the posterior one is so much in the shadow that it does not show well in any of the photographs. The form is, however, essentially like that of Trinucleoides reussi (Barrande), the only other trinucleid of which the hypostoma is known, except that the border does not extend so far forward along the sides, and it is much smaller.

The antennules are not inserted close to the hypostoma, as in Triarthrus, but at some distance from it, and, as nearly as can be determined, directly beneath the antennal pits which are seen near the front of the glabella in many species of trinucleids.

The antennules are long, and are composed of far fewer and longer segments than those of Triarthrus. In this specimen they converge backward, cross each other and at the distal end are more or less intertwined.

As is shown in the drawing and photograph, very little can be learned from this individual about the other appendages of the cephalon. A few fragments of exopodites on either side suggest that these members pointed forward and were much like those in Triarthrus, but nothing conclusive is shown.

The exopodites and endopodites of the left side of the thorax are best preserved. The exopodites are above the endopodites, and only that portion exposed from the ventral side which projects beyond the line at which the endopodites bend backward. The endopodite on the left side of the first thoracic segment is the best preserved. It shows seven segments, the outer ones best. The coxopodite is short and narrow, the basipodite somewhat heavier and longer, while the carpopodite and propodite are the widest and strongest segments. The propodite is triangular and flattened, like the segments on the middle and posterior part of the thorax of Triarthrus. At the inner end of the ischiopodite and meropodite are tufts of spines pointing inward and backward. These are not shown on any of the photographs, but may be seen with the light striking the specimen at the proper angle.

It is not possible to count the exact number of limbs, but one gets the impression that on the left side of this specimen there are twenty-one sets of appendages, six of which of course belong to the thorax. On the thorax and anterior part of the pygidium, successive endopodites show the propodites and dactylopodites becoming progressively more slender and shorter, while the ischiopodites, meropodites and carpopodites become shorter and more triangular, and with increasingly large numbers of short spines on their posterior borders. Back of the fourth endopodite on the pygidium it is not possible to make out the detail, but the appearance is of an endopodite consisting of short broad segments fringed at the back with short spines, the ones at the very posterior end appearing to be exceedingly short and rudimentary.

The exopodites are not so well shown as in some others but the setÆ are flattened and blade-shaped, and often bear numerous small spines.

Measurements: Length (lacking most of the fringe), 10.5 mm. Width of thorax, 10.5 mm. Length of hypostome, 1.41 mm., width at front, 1.46 mm. The distance from back of fringe to end of antennules is 5.4 mm. If straightened out, the left antennule would be about 6.1 mm. long. In the first 3.1 mm., there are only ten segments, so that the average length of a segment is 0.31 mm. The distance from the inner end of the endobase of the first segment of the thorax to the outer end of the meropodite is 2.43 mm., and from that point to the end of the dactylopodite 2.47 mm. making the total length 4.90 mm. These measurements are taken from the photograph. Measurements taken from Professor Beecher's drawing, which was made with the camera-lucida, give a total length of 4.57 mm., the distance to the outer end of the meropodite being 2.3 mm. and thence to the tip of the dactylopodite 2.27 mm. Detailed measurements of the segments, on the photograph, are as follows: coxopodite, 0.321 mm.; basipodite, 0.78 mm.; ischiopodite, 0.68 mm.; meropodite, 0.642 mm.; carpopodite, 0.642 mm.; propodite, 1.01 mm., dactylopodite, 0.825 mm.

Specimen No. 235 (pl. 7, fig. 2; pl. 8, fig. 3; pl. 9, figs. 1, 2).

Specimens 235 and 236 were originally parts of an entire Cryptolithus, but, as Professor Beecher has explained, the specimen was cut in two longitudinally on the median line, and the halves transversely just back of the cephalon, so that each now represents one half of a thorax and pygidium. Both halves have been cleaned from both upper and lower side, a perfectly marvelous piece of work, for the thickness is no greater than that of a thin sheet of paper, and the soft shale of the matrix has a very slight cohesive power.

Both sides of specimen 235 were figured, but the dorsal side was apparently then somewhat less fully developed than at present. On plate 9 are two figures in which specimens 235 and 236 are brought together again, and both dorsal and ventral sides illustrated.

On the dorsal side, specimen 235 shows portions of three exopodites which lie in a direction roughly parallel to the outer portions of the endopodites on the lower side, that is, their direction if projected would reach the axis in an acute angle back of the end of the pygidium. The setÆ stand at right angles to the shaft, and on a portion of it 0.5 mm. long there are seven of them. This is a fragment of an exopodite near the front of the thorax, and the setÆ, which are flattened, are about 1.63 mm. long.

On the ventral side this same specimen shows incomplete endopodites and exopodites of about seventeen segments, six of which would belong to the thorax and the remainder to the pygidium. The greater part of the appendages belonging to the pygidium are exceedingly small (about 0.15 mm. long) and so incompletely exposed that the structure can not be definitely made out.

The endopodites of the thoracic segments all lack the greater part of their proximal segments and are all of practically the same form. They turn abruptly backward at the outer end of the meropodite, and the carpopodite of each is greatly widened, projects inward and is armed with tufts of spines. The propodite and dactylopodite are wide, flattened, and taper but slightly outward, the dactylopodite bearing on its distal end a tuft of spines. On several of the endopodites, the meropodites are visible and they bear on their inner ends fringes of spines pointing inward. Behind these well preserved appendages the proximal segments of several endopodites are visible, and a regular succession of flattened, oval bodies armed with numerous forward-pointing spines. These latter bodies Professor Beecher took to be leaf-like exopodites, which they certainly resemble, and as they lie beyond the line of endopodites they probably do belong to the outer halves of the appendages.

The exopodites under the thorax are long, the shaft shows numerous short segments, and is in each case bent backward, though not through a right angle. They extend considerably beyond the endopodites. The setÆ do not diverge from the shaft at a right angle as on the dorsal side of this same specimen, but at an acute angle, indicating that they were not rigid. The individual hairs are broad and blade-shaped, frequently with a linear depression along the median line, perhaps due to collapse of the internal tube.

Measurements: The greatest length of the fragment in its present state is 5 mm. The dactylopodite of the second endopodite (without terminal spines) is 0.18 mm. long, the propodite 0.23 mm. long and 0.15 mm. wide; the carpopodite is 0.24 mm. long and 0.38 mm. wide. All measurements were made on the photographs.

Specimen No. 236 (pl. 7, figs. 3-5; pl. 9, figs. 1, 2; text fig. 45).

The right half of the same thorax and pygidium as specimen No. 235.

The specimen is cleaned from both upper and lower sides and, the dorsal test being removed, reveals the long blade-like setÆ of the exopodites, each blade being concave along its median line. They are long on the exopodites of the thoracic segments, but become shorter, without, however, any visible change of form on the pygidium. Although the posterior end is not well preserved, one gets no suggestion from a study of this side of the specimens that the exopodites of the posterior end are in any striking way different from those of segments further forward. The tips of some of the setÆ show minute spines, one to each blade.

On the ventral side are a number of endopodites, but they are more fragmentary than those of the other half of the specimen. Some of the exopodites are well shown, the blades being in all cases broken from the shaft. Two of the endopodites of this specimen are of especial interest, as they have interarticular membranes between the last three segments. Professor Beecher made a drawing of one of these which he placed under his pen drawing (text fig. 45).

Measurements: The specimen is 5 mm. long from the front of the second thoracic segment to the end of the pygidium. The setÆ on the exopodites of the anterior thoracic segments are 1.7 mm. long, as exposed from the dorsal side. Some of those on the posterior part of the pygidium, only incompletely exposed, are 0.31 mm. long.

The dactylopodite of the first endopodite showing the articular membranes is 0.23 mm. long and 0.13 mm. wide. The propodite is of the same length and 0.17 mm. wide. The interarticular membrane between them is 0.066 mm. thick. The spines on the dactylopodite of this appendage are 0.15 mm. long. All measurements were made on photographs.

Specimen No. 238 (pl. 8, fig. 4; text fig. 46).

A triangular specimen consisting of the greater part of a pygidium and parts of all the thoracic segments. Under the thorax the specimen has been so cleaned that the outer portions of the endopodites are well shown, while under the pygidium the greater part of the endopodites seem to have been removed, disclosing the setÆ of the exopodites. As in other specimens, the endopodites of the thorax turn backward at the distal end of the carpopodite, which is broad and curved, and bears a tuft of spines on the posterior margin. The dactylopodites seem to preserve their natural shape, and are very nearly cylindrical in form. Under the pygidium are several sets of overlapping fringes of setÆ of exopodites, and along the edge of the dorsal furrow, a number of fragments of segments of what may be coxopodites while with them are a number of fragmentary shaft of exopodites.

Measurements: The pygidium is 3.3 mm. long, the thorax 3 mm.