MUSEUM OF COMPARATIVE ZOOLOGY. 219 



Fig. 3. Another transverse section of the thorax of the same species, showing 

 the mode of attachment of the basal joint of the leg, o ; the simple 

 ribbon-like branchiae ; and the distorted leg on the left side. The 

 branchial ribbon on the right side, I", appears to have been undu- 

 lating and cut across so as to divide it into sections. The section 

 crosses it obliquely, giving a curious outline to the dorsal shell. 



Fig. 4. Transverse section of the extreme posterior end of the pygidium of 

 this species. The section of the terminal spine is shown entire on 

 the left upper side, s, and the base of the other on the right side, h. 

 At X the base of the posterior appendage is shown, and, below, the 

 transverse sections of several of the thoracico-abdominal appen- 

 dages. 



Fig. 6. Longitudinal section of a Ceraurus with appendages very much broken 

 up. At the basal joint of one of the thoracic legs is shown, with 

 the following segments all appearing as one with it. This is not an 

 uncommon mode of occurrence. 



Fig. 8. Longitudinal section of the posterior portion of an individual of the 

 same species. Four appendages are seen connected with the pygid- 

 ium, one for each segment, the one at x corresponding to the one at 

 X. Fig. 4, Plate II. The pygidium has been forced up out of its nor- 

 mal position, but the appendages are seen in another section in the 

 same position, where there has not been any displacement. The 

 appendages of the thorax are mere fragments. The unusual thick- 

 ness of the spar filling the visceral cavity in this and the preceding 

 section is owing to the contents of the visceral cavity having been 

 forced into the curved portion of the shell. This is the usual mode 

 of occurrence in half-enrolled shells of this species. 



Figs. 5, 7. Two longitudinal sections of Calymene senaria, showing the ceph- 

 alic cavity and the anterior portion of the thorax. Many such sec- 

 tions were cut, none of which show segments of the appendages or 

 the structure of the manducatory apparatus. The separation of the 

 appendages nearly to the dorsal shell in Fig. 5 is a curious feature. 



Fig. 10. Transverse section of an enrolled Calymene. The hypostoma is seen 

 at h, and the sections of the legs radiating out from it. The legs 

 are cut across at a different distance from the base of each, and the 

 general form of the leg of this species is taken from this and simi- 

 lar sections, as no approximately entire longitudinal section of the 

 leg of Calymene has been obtained. 



Fig. 9. Transverse section of the thorax of Asa])hus platycephalus. The basal 

 joint of the leg is only to be recognized, 4. Another section of the 

 Bame individual, which was partially enrolled, gives a cross section 

 of nine pairs of legs, and on the interior cast of an Asaphtts, pre- 

 served much as Mr. Billings's specimen is, there are traces of the 

 ba-sal joints or points of attachment to the ventral membrane of 

 nine legs on one side of the median axis of the pygidium. 



