le " PALAEONTOLOGY OF NEW-YORK. 



DISCING OF THE HAMILTON GROUP. 



Disciiia niinuta. 



PLATE L 



Orbimla minula : HAit, Geological Report on the Fourth District, 1843, p. 180. 



Shell minute, subcircular, plano-convex. Dorsal valve moderately con- 

 vex, except near the apex, which is more abruptly elevated ; apex of 

 the dorsal valve excentric, and directed forward. Ventral valve flat ; 

 foramen excentric, with sometimes a slight sinuosity in the margin on 

 that side of the shell. Surface marked by fine crowded and wrinkled 



striae. 



In well-preserved specimens the surface is black and shining, but this 



condition is not always maintained. 



The specimens have usually the greater diameter one- twentieth, varying to three- 

 twentieths of an inch. The species is extremely abundant in some layers of the 

 Marcellus shale, but is not known to have any great vertical or horizontal range. 

 Its extreme minuteness distinguishes it from any other species known to me in the 

 New-York formations. 



Geological formation and locality. In the Marcellus shale : near Avon, N.Y. 



Discina liuniilis ( n. s.). 



PLATE n. 

 Shell of medium size, circular or subcircular, very depressed-convex on 

 the dorsal side ; apex subcentral. Ventral valve flat, with apex sub- 

 central ; foramen apparently submarginal. 

 Surface, from the apex halfway to the margin, marked by fine concentric 

 strice, and outside of this by a few comparatively distant sharp elevated 

 striae, with the intermediate spaces scarcely perceptibly striate. 

 Two specimens only of this species have been recognized : the larger of these 

 has a diameter of more than ^n inch ; and the smaller one, about three-fourths 

 of an inch. They are more nearly circular than any other species in the Marcellus 

 shale and Hamilton group, except the D. minuta. The D. lodensis is sometimes 

 circular ; but its prevailing form is broad oval, and it is always closely and fine- 

 ly striated, and, in this feature, very distinct from the present species. 



