5© PALEONTOLOGY OF NEW-YORK. 
valve is also more gibbous, and the separated valves are conspicuously deeper 
than in 0. vanuxemi. 
This form is comparatively rare, occurring in few localities and having a li¬ 
mited range, while the other has a wide geographical and great vertical range. 
In one locality, great numbers of this species have been found limited to a ver¬ 
tical range of a few inches; while the other species occurs in the beds above 
and below at the same place, with scarcely an individual mingled among these. 
Geological formation and locality. In the shales of the Hamilton group, at 
Eighteen-mile creek, Erie county; on the shores of Canandaigua lake, Ontario 
county, New-York; and Cumberland, Maryland. 
Orthis penelope. 
PLATE YI. * 
Orthis penelope : Hall, Thirteenth Report on the State Cabinet, 1860, p. 79. 
Shell large, oblate, proportions of length and breadth usually as four to 
five, plano-convex; hinge-line about two-fifths the width of the shell; 
cardinal extremities regularly rounded. Dorsal valve regularly convex, 
the greatest convexity about the centre, with a very slight mesial de¬ 
pression or flattening along the centre : beak small, rising but little 
beyond the general outline of the shell; area little more than half as 
wide as the ventral area, not incurved, and lying nearly in a plane 
with the anterior margin of the valve. Ventral valve depressed-convex 
above, sometimes a little gibbous towards the umbo, flat or often con¬ 
cave in the middle and below, the front without sinuosity : area in 
the larger specimens nearly one-eighth of an inch in width; foramen 
broad, twice as wide as high, and nearly filled by the strong cardinal 
process of the opposite valve ; ventral beak obtusely pointed and 
scarcely incurved. 
Surface marked by fine radiating bifurcating striae, which are strongly 
arched upwards near the cardinal extremities, and crossed by fine 
concentric lines, giving a slightly rugose appearance in well-preserved 
specimens, and beside these are closely arranged lamellose lines of 
growth. The radiating striae have often the appearance of being broken 
