MUSEUM OF COMPARATIVE ZOOLOGY. 221 



the spirals were left behind in the mud, and thus dravm out and 

 compressed so that the plane of the section could pass through 

 the si>iral and bring to light its true structure. In Fig. 4 the 

 same thing has occurred, except that the parts are in the pygidium 

 and the spiral form is not as well shown. Fig. 6 shows the spirals 

 towards the head, but in a reverse position from that of Fig. 5. 

 These sections are instructive as showing the strength of the spiral 

 branchiae, and also one of the vicissitudes to which the appendages 

 were subjected, antecedent to their mineralization. 



Fig. 7. Transverse section of an enrolled Ceraurus, illustrating the position 

 of the intestinal canal, and the transverse coiTugations or segments 

 of the ventral surface of the visceral cavity. A few fragments of 

 the branchiae and thoracic legs are shown. 



Fig. 8. Transverse section of an enrolled Cabjmene, showing the branchiae, 

 etc., enclosed within the pleural lobe. 



Figs. 9, 10. Two sections of an enrolled Cab/mene cut from the same individ- 

 ual. Fig. 9 is from the anterior portion of the thorax, and Fig. 10 

 was taken some distance farther back. The bifid spiral branchia 

 is shown in each, as also the basal joint of the leg and the small 

 epipodite (e) or branchial arm. The small appendage at n is 

 probably one of the teinninal swimming joints of the cephalic 

 legs detached and happening to occur in this position. The bran- 

 chia seen on the right side of Fig. 9, nearly touching the dorsal 

 shell, is out of its normal position, as the space it occupies was 

 taken by the membrane uniting the visceral cavity with the margin 

 of the pleurae. With this exception no sections have been cut 

 showing any indication of an attachment of the branchia to the 

 side of the visceral cavity. The large basal joint of the leg is 

 distinctly shown in Fig. 9. The extension of the leg forward 

 would give the section of it seen. The structure of the bifid 

 spiral is better shown by these sections than any others. The 

 base is attached to the basal joint of the leg on the same process 

 with the epipodite. It projects directly outward a short distance 

 and then bifurcates, each branch being nearly as large as the 

 proximal portion. The spiral commences just beyond the bifur- 

 cation and continues nearly to the end, where, in most instances, 

 it terminates in a slightly curved extension of the ribbon forming 

 the spiral. Each branch of the spiral curves outward and doAv-n- 

 ward, just within the pleural lobe, to nearly the margin of the 

 dorsal shell. As shown in the section illustrated by Fig. 4, 

 Plate IV., the coils of the upper portion of the spiral are at- 

 tached to the base or support so as to give it additional firmness. 



