The Nautilus. 



Voi,. XXI. MAY, 1907. No. 1. 



THE DIFFERENCES BETWEEN THE TWO NEW ENGLAND 8PECIES OF 



ACMAEA. 



BY HENRY JACKSON, JR. 



During the summer of 1906, at North Haven, Penobscot Bay, 

 Maine, 1 collected about fifteen specimens of Acmaea testudinalis 

 (Mailer) and thirty Acmaea alveus (Conrad), alveus being by far the 

 more common. I put A. alveus here as a species rather than a var- 

 iety of A. testudinalis, in accordance with one of the latest lists of 

 New England mollusks which gives Conrad's species alveus specific 

 rank. From this material I procured the radulas with the intention 

 of studying the differences between the species testudinalis and alveus, 

 Tbe shell in testudinalis is a roundish and as a rule regular shell, 

 there not being so much variation in form as in alveus. The shell 

 of alveus is a narrower, laterally more compressed shell than that of 

 testudinalis, and is found on eel grass, wharf piles and occasionally 

 on rocks, but at North Haven it is most commonly found on eel grass 

 which grows in great profusion, much to the disadvantage of boats 

 and boatmen, all along the muddy shores. A. testudinalis is com- 

 monly found on flat stones between tide marks. The coloring of tes- 

 tudinalis, as far as observed at North Haven, has much less variation 

 than that of the form alveus, which runs from nearly white to dark, 

 blackish-brown, with many intermediate shades. This narrow, com- 

 pressed form of alveus, it would seem, might have been caused by 

 its being on eel grass which has narrow leaves and might cause a 

 shell to be narrow by the lack of space to grow on. But Mr. Blaney 

 has found alveus at Iron Bound Island, Frenchman's Bay, Maine, 



