•• PAL.EOXTOLOGY OF NEW-YORK,; 



Discina sencea ( n. s). 



PLATE II. 



DoBSAL valve broadly oval ; apex about one-third from the posterior 

 margin, and, in a specimen six-tenths of an inch in length, is elevated 

 one-tenth of an inch above the plane of the margins. 

 Surface marked by concentric, somewhat regular folds or wrinkles in 

 the larger specimens, and, in a smaller one, the surface is similar, but 

 less uneven ; the concentric markings being more like undulations of 

 the surface, than the ordinary striae of Discina. 



Two specimens of dorsal valves only have been seen : they have the same form 

 as D. media and D. lodensis; but the apex is much more elevated, and the surface 

 strias are not of the same fine and regular character, though, the specimens being 

 partial casts, this feature may be somewhat obliterated. One of the specimens has 

 a length of six-tenths of an inch, with a width of five-tenths ; while the smaller 

 one has a length of one-fifth, and a width of one-sixth of an inch. 



These specimens occur in the same shale with the other species, which are al- 

 ways flattened, while both the old and young of this maintain their proportions 

 as described. With the single larger specimen, I hesitated to characterize the 

 species ; but finding a small one of the same form and character, I can scarcely 

 indicate it as a variety of either of the other species which it resembles in general 

 form. 



Geological formation and locality. In the upper part of the Hamilton group : 

 on the east shore of Seneca lake. 



Discina media ( n. s.). 



PLATE II. 

 Compare Dueina lodenili, pa. 22, pi. 2. 



Shell broadly elliptical or subcircular, variable in form. Dorsal valve 

 very depressed-convex ; apex excentric, pointed, and inclined towards 

 the posterior border. Ventral valve flat, or a little convex just anterior 

 to the foramen ; foramen narrow, directly in the longitudinal axis of 

 the ehell, or often a little oblique. 



