VEXERID^. 325 



and it even requires some degree of conchological faith 

 to believe in their existence. The present family abounds 

 in species mostly frequenting southern latitudes. Nearly 

 all of them live in soft ground, buried a couple of 

 inches in gravel, sand, or mud, or among nullipores. 

 A variety of Tapes pullastra, however, inhabits the de- 

 serted holes of Pholades and Saxicavce, as well as the 

 crevices of submarine rocks, wisely accommodating itself 

 to the circumstances in which it happens to be placed. 

 Their bathymetrical range is very extensive, one kind 

 {Venus gallina) inhabiting the shore and the greatest 

 depth ever reached by the dredge. From the investi- 

 gations of Dr. Carpenter into the microscopical texture 

 of their shells, we learn that they are porcellanous and 

 hard, containing scarcely any animal matter. Some 

 kinds are supposed to occur in secondary formations ; 

 but owing to the main feature by which they are known 

 from other bivalves of the same shape being internal, 

 and therefore difficult to ascertain in petrifactions, their 

 geological history is necessarily involved in some ob- 

 scuritv. Their own svstematic relations and divisions 

 into genera are also unsatisfactory. With the excep- 

 tion of Lucinopsis, it is difficult to define any of these 

 genera by precise characters. Deshayes appears to 

 be right in rejecting the claims of Cytherea, and Clark 

 has given equally valid reasons for contesting the sepa- 

 ration of Artemis from Venus. I am not convinced 

 that Tapes has a better right to be considered distinct, 

 unless restricted to two out of the four British species 

 which have been assigned to that genus; but I will 

 retain it provisionally, in deference to other conch olo- 

 gi'sts. The number of genera into which it has been 

 proposed during the past century to distribute the spe- 

 cies of Venus alone, is almost incredible. 



