MUSEUM OF COMPARATIVE ZOOLOGY. 201 
the posterior extremity of the head the cephalic cavity which was filled 
with calespar, had a dark round spot midway between the hypostoma 
and median lobe of the head. A sketch taken after the grinding had car- 
ried the section a short distance back shows the dark spot with the same 
outline as the opening seen in Plate IV. fig. 1, that leads into the intes- 
tinal canal from the cephalic cavity as exposed in the specimen. ‘That 
this was the normal form of the intestinal canalis doubtful, but the 
transverse section, Plate IV. fig. 2, shows the opening in Fig. 1 divided 
into two openings caused in all probability by the ventral membrane 
with its central ridge, having been pressed up against it. In several 
transverse sections a round dark spot is seen in the spar beneath the 
thoracic segments, as in Plate LIT. fig. 7. This was filled with the sedi- 
ment or mud, and thus preserved distinct, โท the section illustrated by 
Fig. 7, the canal is much larger than it is usually found, owing probably 
to distension. The specimen illustrated on Plate IV. fig. 7, shows a 
portion of the dorsal shell of the median lobe broken away so as to 
exhibit the openings in the ventral surface that gave passage to the 
muscles, etc. of the legs, the partitions separating the segments of the 
ventral surface, and the central ridge to which they are attached. This 
ridge, with the partitions and arches in the membrane beneath, would 
give the necessary strength and firmness to form the baseof attachment 
of the numerous ambulatory legs. It would also influence the form of 
the intestinal canal, as has been mentioned, in case it was pressed up 
against it. The position of the opening of the canal in Plate IV. fig. 1, 
and in the section ground away, would indicate that it passed beneath 
the cephalie shield into the cephalic cavity, and then recurved to the 
opening of the mouth. Posteriorly it extended to the extremity of the 
pygidium, as described by M. Barrande. 
The space occupied by the canal and other internal organs is not 
large, as it is contained mostly between the arched median lobe of the 
dorsal shell and the ventral membrane, as shown in the restoration of a 
cross section of the thorax, Plate VI. figs, 2 and 3. The membrane 
uniting the margins of the dorsal shell and the median lobe of the 
ventral surface curves upward close to the plural lobes of the dorsal 
shell, and leaves but a narrow space between it and the dorsal shell 
to be joined to the central cavity. 
Appendages of the Head. — As previously stated, the hypostoma 
has been fully discussed and its various forms illustrated by M. 
Barrande, so that it only remains to mention it as it occurs in the two 
species of Trilobites from which the sections illustrated were obtained. 
