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TRENTON LIMESTONE. 127 



169. 16. ORTHIS OCCIDENTALIS ( n. .?/).) . 



Pl. XXXII. A. Figs, 2 « - m; and Pl. XXXII. B. Figs. I a - i. '^ . 



Resupinate, transversely somewhat oval, or longitudinally scmioval ; length and breadth 

 about as 5 to 7 ; cardinal line equal to the greatest widtii of tlic 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 (ho radii. 



This species, in some of its phases, approaches in general aspect to the last, but diflerg 

 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 striaj are straight - 

 and direct, the last ones not bending upwards as in the O. suhquadrata. 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 iii 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 otiicrs 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. 



Plate XXXII. A. 



Figs. 2 a - f, are illustrations of a series of this species, beginning wiih the smallest recognized speci- 

 mens, and passing througli the several grades, till the increasing rotundity of the ventral 

 valve towards the umbo, rises above the bcnk 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 shoAvn, as is also the slightly incurved beak of the 

 ventral valve ; the latter becoming more gibbous as the shell advances in age. .« . 



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