444 THE ANATOMY OF INVERTEBRATED ANIMALS. 



poda^ and Prosohranchiata, occur in the Silurian formations. 

 Among the Prosohrayichiata^ the Patellidoe and the Asjndo- 

 hrcmchia are the characteristic forms of the older formations, 

 the Ctenohranchia 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 8caphopoda is but 



Fig. 124.—^, Sepia offid7ialis. B, lateral view of the horny ring of an acetabulum. 



little disturbed in this group of the Odontophora. The 

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

 divides the body into corresponding halves ; wdiile the bran- 

 chife, 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 hnsmal 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- 



