METAZOA MOLLUSCA. 213 



the anterior end, while in the adult it is narrower. This 

 indicates a development of the forward or head end of 

 the body. 



Hysteroconcha lupanaria Less. (No. 426), is instructive 

 as showing how age affects the development and increase 

 of spines, ridges, and projecting shelves. The young 

 shell is smooth. In the nepionic stage the spines are 

 short. They appear to be formed by the shell layers 

 meeting along two ridges on the posterior end and being 

 prolonged in such a way as to leave a groove on the 

 upper side of the long tapering spine ; in these grooves 

 foreign particles are often caught. 



A unique specialization of the Pelecypods is found 

 in the Cretaceous Coralliochama (No. 427, C. orcutti 

 White), and Radiolites (PL 428). In Coralliochama 

 one valve is deep and more or less distorted, while the 

 upper valve is convex (see No. 427). In Radiolites the 

 upper valve is flat and serves as a lid or cover. These 

 Pelecypods may be related to the Chamidae. 



Specialization has gone on in Tridacna crocea Lam. 

 (No. 429). As the circular young shell grows older, the 

 wavy lines become ridges and the shell lengthens in an 

 antero-posterior direction. This species in the adult 

 stage is colossal in size and all traces of the young shell 

 are lost. The same tendency is observed in Chama laza- 

 rus Linn. (No. 430) where the originally smooth shell 

 quickly becomes ornamented, the edges of the layers of 

 shell extending into short, broad, flat spines. 



Sections. GASTROPODA. 



The earliest larval stage of existing Gastropods is the 

 trochophore which is so strikingly like the trochophqre 

 of Pelecypods already described that the two probably 

 arose from a common ancestor. As a rule the tiny cap- 

 like shell of the Gastropod becomes a cone, as seen in 



