BULLETIN OF THE 
dages is well shown in all, 4, 4, of the figures, and the first three 
pairs the most distinctly in Figs. 9 and 10 (1, 2, 3, Fig. 10), although 
portions are finely illustrated in Figs. 6 and 8. Figs. 8 and 9 are 
considered as denoting the presence of swimming joints (m, n). 
The organs b, b of Fig. 8 are thoracic, comparable to b, b of Fig. 5. 
In all the sections illustrated on this plate there is no connection 
of the central portion or visceral cavity with the doublure or 
incurved portion of the margin of the dorsal shell. The union of 
the portions of the membrane and enclosed organic substance, re- 
placed by calespar, in Figs. 6 and 8 a, a, and 4, would make such 
a connection complete. In many other sections not illustrated the 
same conditions prevail, so that we know that the space between 
the central mass of appendages and the margin of the dorsal shell 
was united by a thin membrane that left a narrow space between it 
and the dorsal shell. "The suggestion made by Mr. E. Billings, 
that the central mass of legs, etc. of the Trilobite was probably 
supported in the same manner as the same parts in Limulus, is thus 
shown to be correct. 
PLATE II. 
Fig. 1. A transverse section of the thorax of Cerawrus. As is the case with 
the sections of the head, the central mass of the body is not united 
to the margins of the pleure in any of the sections, but the proof of 
such a union in the same manner as in the sections of the head is 
found in portions of such a membrane existing in many sections. 
This section is of an enrolled individual. As the outline of the 
cephalie shield and central portion of the cephalie cavity only are 
shown in the lower portion, the half cutting across the thorax is used 
for illustration. The position and character of the jointed ambu- 
latory legs is beautifully shown, and also the presence of the bran- 
chiæ : the left leg, as in the figure, gives the basal joint and an 
outline of the succeeding joints; this is still more finely shown by 
the leg on the right side. The space occupied by the visceral cavity 
is compressed and filled with calespar. No remains of the intes- 
tinal canal are to be distinguished. 
Fig. 2. A section of the right side of the thorax of another enrolled individual, 
The spar filling the hypostoma is seen at h. The Trilobite was 
broken and the left side lost, so that but one half of the transverse 
section could be obtained. The leg is broken near its base, and also 
towards the extremity. It appears to be formed of seven joints, 
and perhaps eight, if the terminal portion is not a fragment of 
another leg brought into the plane of the section in line with the 
other joints. The branchial appendages were misplaced by the 
movement that carried the leg to the left, as seen in the figure. 
