DISCING OF THE HAMILTON GROUP. 
narrow indentation on the anterior slope ; and in ventral valves in 
similar condition, there is sometimes a lobed muscular impression-^just 
anterior to the foramen. 
This species is very abundant in certain localities of the Genesee slate. The 
oval-ovate forms rarely or never exceed nine-twentieths of an inch in length, by 
eight-twentieths in the greatest width. The largest subcircular form obtained has 
a length a little less than half an inch, with a width of full half an inch in its 
greatest diameter, the posterior end being truncate. 
In form and proportions, this shell scarcely differs from the D. media, hut is 
smaller than the generality of that species, the striae much finer and closer, and 
it possesses the faint radiating folds which do not appear in that species. 
Geological formation and locality. In the Genesee slate, above the Hamilton 
group : near Lodi and Bigstream point, Seneca county, N.Y. 
©isciiia truncafa (n. s.). 
PLATES I & II. 
Shell ovate, the anterior end broader : valves depressed-convex. Dorsal 
valve with the apex near the posterior margin, and directed backwards ; 
posterior margin very abruptly rounded or truncate. Ventral valve 
with the apex submarginal; foramen extending nearly or quite to the 
posterior margin, which is indented. 
Surface marked by fine concentric striae and faint radiating undefined 
lines. 
This little species scarcely ever exceeds two-tenths of an inch in length ; while 
the greatest breadth, which is anterior to the middle, is nearly the same. The 
dorsal valve has the appearance of a Lingula, with flattened margin extending 
beyond the beak : the ventral valve shows a submarginal foramen. The muscular 
impressions (which are faintly preserved ) differ from the ordinary Discina, but 
are too obscure to afford means of separating it from the genus. 
In some specimens the posterior margin is abruptly rounded, while in others it 
is straightly truncate. It may, perhaps, have been an elongated form of this shell 
which is.figured as Lingula concentrica in the Geological Report on the Fourth 
District. 
Geological formation and locality. In the Genesee slate : near Lodi, and at 
Bigstream point, Seneca county; 
