Page:Walcott Cambrian Geology and Paleontology II.djvu/275



The general form is shown by figures 1, 4, and 5, plate 26. The exoskeleton is composed of a strong cephalic carapace (c) (fig. 1, pl. 26) which extends as two long, strong, curved spines (x) that continue posteriorly over the back of the thorax beyond the end of the body. At each antero-lateral angle a strong, backward-curving spine (a') complements the great dorsal thoracic spines. A pair of large crescentiform sessile eyes occur on the anterior margin just within the base of the anterior spines (pl. 25, figs. 4 and 5). The great dorsal spines are beautifully crenulated on the outer margin by short, strong backward-curving spines.

Appendages.—The antennæ (a", fig. 5, pl. 26) are long, slender, and many-jointed; they unite with the head near the posterior end of the labrum; the third pair of appendages, mandibles (m), are large, long, 7 ?-jointed and with fine setæ on the edge of the joints which give the appearance of a slender feather to the appendage (fig. 3, pl. 25, and fig. 3, pl. 26). The fourth and fifth cephalic appendages are slender, and with long joints.

There are 24 pairs of thoracic appendages. Each one is composed of a jointed leg of seven joints with a flattened, short, broad spine on all but the proximal and distal joints (thl, fig. 6, pl. 26); ten legs, (endopodites) with the expanded joints are shown in figure 6, plate 26; anterior to these there are preserved 4 setiferous appendages that appear to be the exopodites of the leg. These are more fully shown by figure 6, plate 25, also figure 3. Another view of the long, jointed endopodite is found in figures 3 and 5, plate 26. The presence of a gill (epipodite) was unsuspected until the specimen represented by figure 4, plate 26, was found; this fine example is so delicate and so beautifully preserved that it is almost unique even among the wonderful Burgess shale fossils.

Abdomen.—The abdomen (ab, figs. 1, 3, 4, and 6, pl. 26) forms a small, plate-like termination of the long body.

Interior structure.—The alimentary canal is very distinct in a number of specimens (i, figs. 3 and 6, pl. 26). It extends from the posterior margin of the labrum back to the plate-like abdomen. The head is too much smashed down to show any details of interior structure, but the alimentary canal appears to widen out and occupy much of the space beneath the subquadrangular carapace. The large dorsal spines have a central canal that appears to open into the space (stomach ?) beneath the carapace; this canal may represent the