48 CONTRIBUTIONS 



Isle of Wight, and three from the London Clay, of which 

 the bed from which the above were obtained is the equi 

 valent in Alabama. Eighteen species in all have been 

 observed in the various formations of Great Britain. In 

 the Tertiary Tables of M. Deshayes we have thirty-five 

 species, twenty-one of which are found in the Paris basin 

 (the Eocene). In this country Mr Say has described 

 three from the Tertiary of Maryland, and Mr Conrad one 

 from the Tertiary of Claiborne. 



FAMILY LITHOPHAGA. 



GENUS BYSSOMIA. Cuvier. 



B. petricoloides. Plate 1. Fig. 16. 



Description. Shell subcylindrical, very thin, very trans 

 verse, indistinctly striate, on the posterior part obliquely 

 and obsoletely folded ; beaks scarcely perceptible ; pos 

 terior and anterior cicatrices perceptible. 



Length .1, Breadth .3, of an inch. 



Observations. With much doubt I have placed this shell 

 in Cuvier s genus Byssomia. It agrees better with his 

 description of that genus than any one I know.* Unfor 

 tunately I obtained but a single valve, and that is imper- 



* He says the shell is oblong, and has no &quot; marked tooth,&quot; and that it 

 penetrates into stone, coral, &c. 



