36 PALEONTOLOGY OF NEW- YORK. 



The interior of the ventral valve preserves very strong dental lamellte, 

 and a strong callosity for the attachment of the cardinal muscle. 



I refer this species to the 0. lenticularis of Vanuxem with some little doubt, 

 since the original specimen in the State Collection has been lost. The form de- 

 scrilicd by Mr. Vanuxem is similar to that of figs. 1 and 2 of Plato v, which is 

 subquadrato, the front being sometimes rounded and sometimes straight. The 

 other forms ( figs. 3, 4 and 5), which I find myself compelled to refer to the same 

 species, are more robust, with the greatest width below the middle, and a much 

 greater convexity of both dorsal and ventral valves. 



In form, this species resembles the 0. subcarinata of the Lower Helderberg 

 group ; but the strias are finer, the area larger, and the muscular callosity and 

 dental lamellae are stronger ; while the narrowing towards the hinge in the more 

 robust forms is a character not observed in that species. 



The specimens ordinarily seen have a length and breadth of a little more or 

 less than half an inch, with a depth of about one quarter of an inch ; while the 

 largest specimens have a length and breadth of three-fourths of an inch, with a 

 depth of about half an inch. 



Geological formation and locality. In the Corniferous limestone of the Upper 

 Helderberg group : at Leroy, Genesee county; near Caledonia, Livingston 

 county ; two miles south of Vienna, and at Shortsville, Ontario county, N.Y. 



B. Species of the type of Orthis hybrida and O. oblata, with the dorsal valve more 



convex than the ventral : 

 Orthis alsus, Orthis ( sp. ? ), 



Orthis mitis, Orthis vanuxemi? 



Orthis livia, Orthis semele, 



Orthis cleobis. 



Orthis alsas ( n. s.). 



PLATE IV. 



Shell somewhat semielliptical in outline : cardinal line equal to, or 

 more than two-thirds the greatest width, which is near or just below 

 the middle. Dorsal valve more convex than the ventral, the greatest 

 convexity being about the middle, sloping almost equally to the sides 

 and front, and less rapidly to the beak, while it is flattened or concave 

 towards the cardinal angles. The centre is marked by a sinus from the 



