TRENTON LIMESTONE. 127 
169. 16. ORTHIS OCCIDENTALIS (xn. sp.). 
Pu. XXXII. A. Figs, 2 a-—m; and Puy XXXII. B. Figs. 1 a -7. 
Resupinate, transversely somewhat oval, or longitudinally semioval ; length and breadth 
about as 5 to 7; cardinal line equal to the greatest width of the shell; area large, triangu- 
lar, partially common to both valves ; foramen narrow, triangular, reaching to the apex of 
the dorsal valve ; dorsal valve convex towards the beak, and usually flattened or slightly 
convex towards the margin (in old shells a broad depression in front) ; beak much elevated, 
straight, not incurved; ventral valve regularly convex, with a slight depression along the 
centre ; beak slightly projecting beyond the cardinal line, and incurved ; surface marked 
by subangular radii, which bifurcate at one-half or two-thirds the distance from beak to 
base ; radii crossed by fine sharp elevated concentric lines, which are usually well preserved 
in the spaces between the radii. 
This species, in some of its phases, approaches in general aspect to the last, but differs 
in essential particulars. The length from beak to base is proportionally less than in the 
last ; the depth of the two valves together, when not compressed, is greater ; the beak of 
the dorsal valve is more elevated, the area larger and foramen longer; the beak of the 
ventral valve is likewise a little more incurved ; the radii are stronger, and do not bifurcate 
near the beak ; the concentric elevated lines are sharper and finer ; the striz are straight 
and direct, the last ones not bending upwards as in the O. subquadrata. As the shell be- 
comes advanced, the dorsal valve presents an increasing depression towards the margin, 
which finally becomes a broad, not distinctly defined sinus. At the same time the slight 
depression in the centre of the ventral valve, similar to that in the last species, does not 
reach the margin, and finally becomes obsolete. The slight elevation in front, shown in the 
last, is exactly reversed in this species. 
These characters, when once observed, will not fail in enabling the student to identify 
the species, and to distinguish it from any others in the same geological position. 
The internal structure is not as well known as in the last, the interior of the dorsal valve 
not having been seen. The interior of the ventral valve corresponds in general character to 
the last; the small medial ventral tooth does not, however, reach as high as the plane of 
the area, and it is thin and sharp, while the last is thick. The interior surface is marked 
nearly to the beak with the impressions of the external radii; while in the last, these 
markings reach only a short distance from the margin. 
Puate XXXII A. 
Figs. 2 a —f, are illustrations of a sertes of this species, beginning wiih the smallest recognized speci- 
mens, and passing through the several grades, till the increasing rotundity of the ventral 
valve towards the umbo, rises above the beak of the dorsal valve. 
Figs. 2 g, h. Profile views of several specimens as above. The strong, nearly straight, and greatly 
elevated beak of the dorsal valve is well shown, as is also the slightly incurved beak of the 
ventral valve; the latter becoming more gibbous as the shell advances in age. 
