| 
| 
| 
MUSEUM OF COMPARATIVE ZOOLOGY. 221. 
the spirals were left behind in the mud, and thus drawn out and 
compressed so that the plane of the section could pass through 
the spiral and bring to light its true structure. In Fig. 4 the 
same thing has oceurred, 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 
branchiæ, 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 corrugations or segments 
of the ventral surface of the visceral cavity. A few fragments of 
the branchi and thoracic legs are shown. 
Fig. 8. Transverse section of an enrolled Calymene, showing the branchiæ, 
Figs. 9, 10. 
etc., enclosed within the pleural lobe. 
Two sections of an enrolled Calymene 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 terminal 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 pleure. 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 
distinotly 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 down- 
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, 
