86 POPULAR CONCHOLOGY. 



teeth. They are generally orbicular, and 

 closed all round. Three of the genera are 

 fluviatile, have lateral teeth, and are covered 

 with an epidermis. The rest are marine, and 

 have for the most part no lateral teeth or epi- 

 dermis. There are seven genera. 

 43. Cyclas. — Shell thin, roundish, or trans- 

 verse, equivalve, and inequilateral, 

 two small cardinal teeth in each 

 valve, one of which is folded in two, 

 but rather uncertain, lateral teeth 

 lengthened transversely, and compressed, liga- 

 ment exterior ; an epidermis. Animal, foot 

 large. — 11 species recent. 



The shells of this genus are not larger than 

 a hazel-nut, and some of the species are very 

 thin and transparent, striped transversely with 

 light colours. They inhabit rivers in various 

 parts of Europe and America, and have pro- 

 jecting, not eroded bosses. 



44. Cyrena. — Shell thick, in- 

 equilateral, and round in form, 

 cardinal teeth three in each valve, 

 lateral generally two, ligament 

 external, and on the longest side, 



