TRENTON LIMESTONE. 127 



169. 16. ORTHIS OCCIDENTALIS ( n. sp.) . 



Pi.. XXXII. A. Figs, 2 a - m; and Pi,. XXXII. B. Figs. 1 a - i. '^^ . 



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



V 



■*• 



■,-» 





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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 striae are straight 

 and direct, the last ones not bending upwards as in the 0. 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. *ir i* ' 



These characters, when once observed, will not fail in enabling the student to identify ' ^-' 



tlie 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 ^jk 

 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 mterior 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 illustralions of a series 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. 



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