44 
PALEONTOLOGY OP NEW-YORK. 
area inclined to the area of the opposite valve, and about one-half or 
two-thirds as wide ; dorsal beak full, broad and rounded or obtusely 
pointed, extending a little beyond and slightly incurved over the area. 
Ventral valve less gibbous than the dorsal, very prominent on the 
umbo, sloping very abruptly to the cardinal angles and more gently 
towards the lateral and baso-lateral margins, flattened in the centre ; 
the lower half marked by a gradually increasing, broad, and undefined 
sinus, which gives a strongly curved outline to the front margin : beak 
more acutely pointed and incurved; area elevated and incurved, its 
length equalling or greater than half the width of the shell. The beaks 
of the two valves are approximate. 
Surface marked \>y fine unequal striae which increase by interstitial 
additions, and are crossed by fine equal concentric striae, and, at un¬ 
equal intervals, by stronger imbricating lamellose lines of growth. In 
certain conditions of the surface, the striae show tubular openings at 
the concentric lamellose lines. There are from eight to ten of the larger 
striae in the space of two lines ; and by counting the finer intermediate 
striae, there are sometimes from thirteen to eighteen in the same space. 
The cast of the dorsal valve shows a subquadrate or quadrate-ovate 
muscular scar, with a deeply marked border and longitudinal median 
groove : a groove passes from the centre of the median line obliquely to 
the lower lateral angles, dividing the adductor muscular scar. The va¬ 
scular impressions radiate from the muscular impression in broad bands, 
dividing several times before reaching the margin of the shell. 
This species approaches so closely the 0. multistriata of the Upper Pentamerus 
limestone of the Lower Helderberg group, that it is very difficult to distinguish 
them, the form and surface charactess being generally almost the same. Some¬ 
times, however, the ventral beak and area in this species is a little more arcuate 
than in 0. multistriata, and the dorsal valve is usually more gibbous, while in¬ 
ternally they present well-marked differences. In this shell, the vascular im¬ 
pressions bifurcate once, twice, or even three times, before reaching the border ; 
while in 0. multistriata , they pass down the front without bifurcation. 
The Orthis tulliensis of the Tully limestone is another form very difficult to 
be distinguished from this one; but it is generally more gibbous, and presents 
internal differences; the divisions of the vascular impressions pass down the 
