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. Cwier. 
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. 
Diam..... 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 “ marked tooth,” and that it 
penetrates into stone, coral, &c. 
