1869.] 147 [Perkins. 



JlEAStJREMEftTS. 



Length of shell 80 mill. Breadth 55 mill. 



11 foot 21 " (contracted). " 24 " 



u gills 45 " " 20 " 



" palpi 31 " " 7 " 



" siphons 10 " " 11 " 



After comparing specimens from several localities, I am convinced 

 that the si?nilis Say, is identical with this species, not its young, as 

 some suppose, for the characters of Say's species are found in speci- 

 mens of large size, but simply a variety due to difference of station. 

 I have not yet seen specimens answering to both species from the 

 same locality. Those from Chelsea Beach have the flatter, less tri- 

 angular form ascribed to solidissima, those from here all have the 

 triangular form of Say's similis, while those from Coney Island, N. Y., 

 are so nearly intermediate as to make it impossible to refer them 

 satisfactorily to either. The young of all are more trigonal than the 

 old, but difference in station seems to make great differences in the 

 shell. The British species most nearly allied to solidissima, M. 

 solida, according to Clark, " varies in shape in every locality," l and 

 the same seems to be true, in some degree at least, in respect to our 

 own. The slight differences mentioned as distinguishing the two 

 species aside from the form, are found in nearly all that I have seen, 

 and do not coincide with any particular form. 



VENERID.3E. 



Cytherea Lamarck, 1805. 



Cytherea Sayii Conrad. Cytherea convexa Gould, Invert. Mass., 

 p. 34, fig. 49; De Kay, Moll. N. Y., p. 216, pi. xxvii, fig. 279. Cal- 

 lista convexa Say, Journ. Acad. Nat. Sc. Phil., Vol. iv, p. 149, pi. 

 xii, fig. 3, 1824. 



Very rare. Mr. Say's specific name " convexa " being, as Mr. 

 Conrad says, preoccupied, he proposes the above specific name, 



. Crassivenus nov. gen. 



Crassivenus mercenaria nobis. Mercenaria violacea Stimps., 

 Check List. Venus mercenaria Linn., Syst. Nat., 1131; Lam., An. 



iBrit. Mar. Test. Moll., p. 105, London, 1855. 



