252 NEW YORK STATE MUSEUM 



M. champlaini ordinarily has less sloping cardinal margins 

 and it is uniformly a proportionally broader shell. The convexity 

 of the valves is persistently greater and specially so in late stages. 

 Like M. 1 a e v i s it carries a medial ventral sinus and a dorsal 

 median ridge with depressed lateral slopes, but the linguate ex- 

 tension at the margin is greater. On the interior the vascular 

 markings are highly developed. 



M. arcuata Hall [see Pal. X. Y. 1859. 3:249, pi. 41, fig. ia-t; 

 Hall & Clarke, idem. 1894. v. 8, pt 2, pi. 43, fig. 1, 2; pi. 44, fig. 5] 

 has a much larger umbonal angle than M. 1 a e v i s and this is well 

 expressed in M. c h a m p 1 a i n i. It likewise has the deeper valves 

 of the latter, the ventral being specially curved at the umbo and 

 arched at the beak. Here too we mark the deep lingua on the 

 front margin as large or larger than in the Grande Greve shell. 

 Differences in the two species are obscure but on the whole M. 

 arcuata is less elongate, less sharply ridged on the dorsal valve 

 and the interiors are without vascular markings. 



M. subquadrata Hall [see Pal. N. Y. 1859. 3 :2 49> pi- 4°- 

 fig. 3] expresses a condition in which the form of the shell is 

 squared by the truncation (casual?) of the antelateral margins, 

 making the median dorsal ridge quite prominent, a condition which 

 is sometimes approached accidentally by M. champlaini. 



M. lata Hall [Pal. N. Y. 1859. 3:431, pi. 101, fig. 3a-w], an 

 Oriskany shell, differs from M. arcuata chiefly in size and the 

 tendency to acquire a breadth unusual to that species. The pro- 

 longation of the anterior margin and the depression of the ventral 

 umbo are also distinctive features and in both of these respects the 

 shell is not the same as that in hand. 



M. vascularia (described as M. ? vascularia Clarke, 

 N. Y. State Mus. Mem. 3. 1900. p. 45, pi. 6, fig. 12-14) com- 

 prises large shells with the proportions of M. lata but having the 

 pedicle scar greatly developed and bounded by high dental plates, 

 and the large adductor scar common to all these species obscured 

 almost to obliteration by the pallial ridges and sinuses. The latter 

 are here much more highly developed than in M. champlaini 

 where the muscle scar suffers no obscuration therefrom. 



M. champlaini is the designation which, in view of the 

 peculiarities mentioned, we propose for the shells under considera- 

 tion. It serves to express the fact that they share the features 

 of a series of essentially contemporaneous forms in the American 

 province and at the same time combine these in such a way that, 



