110 POPULAR CONCHOLOGY. 



four genera ; they have all more or less the 

 shape of the common genus, and some are 

 pearly in the inside. Their habits vary, some 

 attaching themselves by a byssus to the rocks, 

 whilst others perforate them ; in all the liga- 

 ment is external, and they have rarely any 

 teeth. 



£ 68. Mobiola. — Shell rather transverse, equi- 

 valve, inequilateral ; bosses pro- 

 minent and slightly removed 

 from the end ; valves gaping a 

 little ; no teeth ; ligament al- 

 most internal. Animal, foot 

 tongue-shaped. — 27 species recent, and 20 

 fossil. 



Attached by a byssus to the rocks, of which 

 they sometimes almost cover the surface. 

 Sometimes found in holes of stones, madre- 

 pores, &c. They are met with in the Indian 

 Seas, New Holland, the Atlantic, &c. 



69. Lithodomus (Cuvier). — Shell trans- 



