ARCA. 179 



time to suppose that the animal excavated rocks ; and 

 he gave this shell the specific name ofperforans, believ- 

 ing that it was not the A. lactea of Linne. The shell 

 varies considerably in the proportion of its different 

 parts, as well as in the comparative tenuity of sculpture. 

 It never grows to much greater dimensions than I have 

 stated in the description. My largest specimen, which 

 was evidently a veteran, and must have outlived most 

 of its generation, is not much more than three-quarters 

 of an inch in length. 



I should have been inclined to consider the present 

 species the A. modiolus of Linne, if it were not for the 

 expression that it is exactly the shape of Mytilus mo- 

 diolus and the size of a large bean. The rest of his de- 

 scription agrees with it in every particular. He even 

 placed A. modiolus in one section as having a plain 

 margin, and A. lactea in another as having a notched 

 margin. He says both inhabit the Mediterranean, and 

 that A. lactea is " diaphana," which is certainly not 

 the case in our shell. In all probability his A. lactea is 

 the A. imbricata of Poli, a thinner shell and having 

 the inside margin strongly notched. Brocchi applied 

 Muller's name of nodulosa to the present species, be- 

 cause A. lactea was described by Linne as possessing 

 the last-mentioned character ] and for the same reason 

 Poli, Olivi, Chierighini, and Costa adopted the name of 

 A. modiolus. But I will not venture to expunge the 

 generally received name of lactea, and to substitute for 

 it another which is referred by many conchologists to a 

 common West-Indian shell especially as so much ob- 

 scurity still involves several of the Linriean species, 

 notwithstanding the laborious research devoted to the 

 subject by Mr. Hanley. What we call A. lactea may 

 be the A. barbata of Miiller's ' Prodromus/ although 



