MUSEUM OF COMPARATIVE ZOOLOGY. 201 



the posterior extremity of the head the cephalic cavity which was filled 

 with calcspar, 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 canal is 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 III. fig. 7. This was filled with the sedi- 

 ment or mud, and thus preserved distinct. In 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 base of 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 cephalic 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. 



