444 THE ANATOMY OF INYERTEBRATED ANIMALS. 



poda, and Prosohranchiata, occur in the Silurian formations. 

 Among the Prosohranchiata, the Patellidm and the Aspido- 

 hranchia are the characteristic forms of the older formations, 

 the Ctenobranchia appearing later, and acquiring their pres- 

 ent relative abundance only in the later secondary and the 

 tertiary epochs. 



The Cephalopoda. — The bilateral symmetry which is so 

 obvious in the Polyplacophora and the Scaphopoda is but 



Fig. 124.— a. Sepia officinalis. B, lateral view of the horny ring- of an acetabulum. 



little disturbed in this group of the Odontophora. The 

 mouth and the anus are situated in the median plane, which 

 divides the body into corresponding halves ; while the bran- 

 chiae, two or four in number, are disposed symmetrically on 

 each side of this plane, as are' the brachial prolongations of 

 the margins of the foot. The hfemal face of the body, how- 

 ever, is not flat, as in the mollusks which have just been men- 

 tioned, but is elongated perpendicularly to the neural face, so 

 as to form a sort of sac, invested by the mantle. On the pos- 



