176 NEW YORK STATE MUSEUM 



Into immediate comparison with these shells, then, we bring the follow- 

 ing known forms : Meristella laevis, M. arcuata, M. -subquad- 

 rata of the Helderbergian fauna; M. lata and M. vascularia of 

 the Oriskany fauna. Comparisons therewith are introduced seratini. 



Meristella laevis Vanuxem (A t r y p a laevis Vanux. Rep't 

 3d Dist. N. Y. 1842, p. 120, fig. 2; M crista laevis Hall, Pal. N. Y. 

 1859, 3 : 247, pi. 39, fig. 3, 4 ; Meristella laevis Hall & Clarke, Pal. 

 N. Y. 1894, V. 8, pt 2, pi. 43, fig. 3-6) is a rather elongate shell with long car- 

 dinal slopes making a relatively small angle with each other. Both valves 

 are moderately and subequally convex, the ventral valve faintly sinuate 

 medially, in old stages with a broad and rather short linguate extension on 

 the front margin which, when slightly broken, as in most of Hall's figures, 

 gives the front a subtruncate appearance. 7'he dorsal valve has a broadly 

 defined median ridge in all stages, but obscure near the margin in the adult. 

 On the interior there is seldom any trace of vascular impressions departing 

 from the muscular area. 



Meristella champlaini ordinarily has less sloping cardinal mar- 

 gins and it is uniformly a proportionally broader shell. The convexity of 

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

 laevis it carries a medial ventral sinus and a dorsal median ridge with 

 depressed lateral slopes, but the linguate extension at the margin is greater. 

 On the interior the vascular markings are highly developed. 



Meristella arcuata Hall \see Pal. N. Y. 1859, 3-249, pi. 41, 

 fig. la-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. laevis and this is well expressed 

 in M. champlaini. 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. 



Meristella subquadrata Hall [.y^^ Pal. N. Y. 1859, 3 : 249, pi. 

 40, 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. 



Meristella lata Hall [Pal. N. Y. 1859, 3: 431, pi. loi, fig. 3a-w], 

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

 ency to acquire a breadth unusual to that species. The prolongation 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. 



Meristella vascularia (described asM. Pvascularia Clarke, 



