36 

 PETRICOLA. 



GENERIC CIIARACTER. 



Shell oblong or subtrigoiial, inequilateral, posterior 

 side narrowed and gaping ; hinge with two or three 

 teeth in one valve^ and one or two in the opposite. 



OBSERVATIOXS. 



Lamarck includes this genus in his family Litho- 

 phaga, on account of the species forming their habi- 

 tations in rocks and stones. Some naturalists have 

 supposed these perforations to be effected by a me- 

 chanical action of the valves, while others agree that 

 they are formed by the secretion of a peculiar acid, 

 acting as a solvent upon the calcareous rocks into 

 which the animals bore. The only species known 

 to inhabit our coast forms a singular exception to the 

 the general habits of this family, as it burrows ex- 

 clusively in hard ground, among the roots of plants. 

 Vcneriipis differs from the present genus merely 

 in having the cardinal teeth minute, and in the spe- 

 cies being of a suborbicular form. Blainville consi- 

 ders these characters too unimportant to warrant the 

 separation of shells so nearly allied ; accordingly in 

 his Malacologie we find them constituting one genus, 

 to which he has retained the name of Vcneriipis. 



