IS 
PALAEONTOLOGY OF NEW-YORK. 
DISCING OF THE HAMILTON GROUP. 
Discina iiiinuta. 
PLATE I. 
Orbicula minuta : Hall, 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. Yentral 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 
Marcel lus 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 v formations. 
Geological formation and locality. In the Marcellus shale : near Avon, N.Y. 
Discina humilis (n. s.). 
PLATE II. 
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 
striae, 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 an 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. . 
