MUSEUM OF COMPARATIVE ZOOLOGY. 203 
Plate I. figs. 8 and 9. If the section passed through these flattened 
joints obliquely, or transverse to their minor axis, a slender leg would be 
shown; if through the major axis, a broad swimming leg would be pre- 
sented. No traces of spines or serrated margins on the inner margins 
of the basal joints have been observed. 
Good longitudinal sections of the cephalie appendages, in a longi- 
tudinal section of the Trilobite, have not been obtained. Usually the 
appendages have the appearance shown in Plate II. figs. 5 and 7, and 
Plate V. figs. 1, 3, and 4. 
Thus far our description of the cephalic appendages has been derived 
from sections of Calymene. In Cerawrus, however, we have illus- 
trations of the same, but in a more fragmentary condition, Plate I. 
figs. 2, 3, and 4. These and many other sections go to show that the 
arrangement of the parts is about the same in each species, — the 
more shallow dorsal shell of Cerawrus necessarily giving a different 
appearance to the section from one cut at the same place and angle in 
Calymene. As yet no other appendages have been observed beneath 
the head that prove to have belonged there. Fragments of the thoracic 
legs and branchiæ are frequently seen in sections crossing the head, 
but they have been pushed forward and are of accidental occurrence. 
Appendages of the Body. — The appendages of the body or the 
thoracico-abdominal legs and branchiæ are found to vary slightly in 
the genera Calymene and. Ceraurus, as expressed by the species under 
consideration. The legs of the former are relatively shorter and more 
symmetrical as compared with the long and somewhat irregularly 
jointed legs of the latter. 
. The Calymene is frequently found enrolled, the head and pygidium 
fitting closely together, so that no opening is left at any point, the legs 
being all drawn within the shell and entirely protected from injury 
from without. With Ceraurus, i.e. in the species under considera- 
tion, a perfect closing of the shell by enrolment is impossible, and the 
space formed by the partial enclosure of the spinous extension of the 
pleura affords but an incomplete protection to the numerous legs and 
branchiæ, 
The finest illustration of the legs of Ceraurus, and of the Trilobite 
as far as yet known, is given in Plate IT. figs. 1, 2, and 3. In Fig. 3 
the form of the transverse section of the basal joint and its mode of 
attachment to the ventral surface are shown. By Fig. 2 the joints of 
the leg are shown, and Fig. 1 adds to our knowledge of their shape 
