MOLLUSCA OR UNIVALVES. 251 



the examples of this genus differ materially in 

 thickness of substance. 



118. Volvaria. — Shell oval ; spire scarcely 

 visible ; mouth narrow, and as 

 long as the shell; plaits few, at 

 the lower part of the inner lip ; 

 outer lip thin, or only slightly 

 thickened. Animal unknown. — 

 5 species recent, and 1 fossil. 



These shells are found on the 

 coasts of Africa and China, where the natives 

 string them into necklaces. 



Family 15. — Convolutidce* 



Shell without a canal, but having the base 

 notched ; the whorls of the spire large, rolled 

 round each other, so that the last nearly, and in 

 some instances entirely, conceals all the others, 

 rendering the aperture long and narrow ; the 

 body of the animal is therefore very flat. The 

 three first genera of this family have the right 

 lip recurved inwardly in the adult. They do not 

 possess an operculum. There are seven genera. 



* Les Enroulees, Lamarck. 



