148 ARCID.E. 



The difficulty arising from the sterility of hybrids among 

 unisexual animals being thus removed, how does it 

 happen that so many distinct but allied species assem- 

 ble and live together without interbreeding or mingling 

 their respective races ? There is no fusion, there are 

 no intermediate forms or gradations from one to an- 

 other; each pursues its own course and mode of life, 

 and appears to have no communion with its neighbour. 

 Surely this emboldens naturalists to maintain the in- 

 tegrity of species as at present existing, however much 

 they may have changed in the course of bygone ages. 

 We take Nature as it is and apparently has been — not as 

 it might have been ; and all speculations as to the origin 

 of species, although ingenious and interesting, are useless, 

 for want of sufficient data to guide us in the inquiry. 



The variety tumidula resembles N. nitida in shape ; 

 but the surface is not glossy as in that species, and the 

 sculpture is the same as in N. nucleus. 



The present species seems not to be liable to distor- 

 tion ; but a valve now before me from Guernsey has a 

 fold on the posterior slope, contracting the shell, and 

 somewhat resembling the sinuosity of Axinus flexuosus. 

 Specimens from the Hebrides and the south-west of Ire- 

 land are larger than any I have seen from other parts, 

 whether British or foreign. Besides the characters above 

 noticed in the description of this species, it differs from 

 N. sulcata in sculpture, in the beaks being more promi- 

 nent, the hinge-line more curved, and the cartilage-pit 

 shorter and broader, in having more teeth on the ante- 

 rior side, and in the crenulations of the inside margin 

 extending further towards the posterior angle. 



It is the Area margaritacea of Bruguiere, and Gly- 

 cymeris argent ea of Da Costa. The fry is the Nucula 

 argent ea of Brown. 



