Fig, 
Fig. 2. 
Fig. 5. 
Fig. 6. 
Fig. 9. 
BULLETIN OF THE 
PLATE IV. 
The head of Cerawrus, with the dorsal shell broken away over the 
cephalie cavity, showing a cast of the interior and the enlarged 
opening of the intestinal canal. 
A transverse section of Fig. 1 across the third thoracic segment. 
The section of visceral cavity and intestinal canal are the only 
traces of parts other than the dorsal shell. The light spot in the 
centre of each dark spot represents the light shining through from 
the front. The division of the intestinal canal into two parts 
is undoubtedly of accidental occurrence. 
A detached thoracic branchia from the interior of an enrolled Caly- 
mena. 
Same from a Ceraurus, showing the manner in which the upper 
portion of the spiral is strengthened by an attachment to an arm- 
like support. 
The original specimen from which the interior cast is sketched was 
80 badly broken that the outlines of the dorsal shell were taken 
from another specimen, aud the break in the shell and interior 
from the original. The figure explains itself, as the cast of the 
basal joints of the legs and the openings leading iuto the legs is 
seen, and also the divisions of the ventral membrane leading to 
the central ridge. 
The interior surface of the dorsal shell of Cerawrus, showing the 
hypostoma in position and the very irregular surface of the thorax. 
The bifurcation of one of the cephalic spines has not been noticed 
in any other Trilobite, bearing spines, that has passed under my ob- 
servation. It is analogous to the bifurcation of the terminal spine 
of Limulus. The two small, oval, dark spots on the inside of the 
first segment of the pygidium are always present in well-preserved 
specimens. They indicate a depression, and it may be an opening 
through the shell, but the exterior shows no trace of them.* 
Median longitudinal section of Ceraurus, The dorsal shell and hypo- 
stoma are alone preserved in the section. The line of the in- 
testinal canal, 4, and the ventral membrane, vm, are drawn to 
show their position as far as has been determined by the sections. 
Third and fourth thoracic segments of Cyamus Scammoni Dall, This 
is introduced to show the peculiar form of the branchia, b, 6, for 
comparison with the spiral branchiæ of the Trilobite. Dr. Dall 
describes the branchiæ as follows: “ The third and fourth seg- 
ments each have a branchia attached on each side. This, near 
the base, divides into two cylindrical filaments spirally coiled 
* For a detailed description of this figure see Annals New York Lyceum Nat. 
Hist., XI. pp. 159-169, 1875. 
