240 



MOLLUSCA. 



Fig. *<yi.Clavagella cretacea. Chalk. 



hinge or ligament, often with accessory valves. Animal club- 

 shaped or worm-like, with a 

 short tnmcated foot. Mantle 

 closed in front ; siphons long, 

 united to near their extremi- 

 ties. 



In the genus Pholas the 

 shell is cylindrical or oval, 

 the valves are edentulous, and 

 there is no ligament or a rudi- 

 mentary one. The pallial 

 sinus is very deep, and the 

 dorsal margin of the shell is 

 protected by accessory valves. 

 The Pholadcs inhabit burrows 

 which they form for themselves in clay, peat, or rock. Many 

 fossil species of the genus are known, commencing in the 

 Jurassic Rocks. 



In the genus Teredo the shell is " globular, open in front 

 and behind, lodged at the inner extremity of a burrow partly 

 or entirely lined by shell ; valves three-lobed, concentrically 

 striated, and with one transverse furrow ; hinge-margins re- 

 flected in front, marked by the anterior muscular impressions : 

 umbonal cavity with a long curved muscular process." (Wood- 

 ward.) Species of Teredo occasionally reach a very large size, 

 and they are known in the fossil state both by their shells and 

 by their burrows in wood. The genus seems to have com- 

 menced in the Lias, and is well represented at the present 

 day. 



CHAPTER XXI. 

 GASTEROPODA. 



THE Gasteropods are Molluscs in which the body is furnished 

 with a distinct head, and the mouth is provided with a mastica- 

 tory apparatus or " lingual ribbon" Locomotion is effected by 

 means of a broad, horizontally-flattened, ventral disc the "foot" 

 or by a vertically-flattened, fin-like modification of the same. 

 The body is never included in a bivalve shell ; and may be naked. 

 Usually, h(ni<ever, tJiere is a " univalve " shell, or in some cases a 

 " multivalve " shell. 



This class includes all those Molluscous animals which are 



