350 PALEONTOLOGY OF NEW-YORK. 



Rli}iichoiicIla (Stcnocisma) doplicata. 



PLATE LV. 



jitrypa dupKeata : Hah, Report on Fourth Gf«ologic8l District of New-Tork ; Fossils of the Chemung 



group, Illustration Ixvii, figs. 2 & 2 a. 1843. 



Shell depressed-suborbicular or subquadrate j length and width about 

 equal : valves subequally convex. 



Ventral valve convex on the umbo, with beak strongly incurved ; sides 

 moderately convex, sometimes a little flattened at the baso-lateral 

 margins and very abruptly incurved on the cardinal slopes : mesial 

 sinus beginning near the apex and becoming deep and angular below 

 the middle, usually marked by a single plication, and much produced 

 in front. 



Dorsal valve convex, most prominent about the middle or below the 

 middle of its length, curving to the sides and produced in nearly a 

 direct line to the front. 



Surface of the dorsal valve usually with two prominent plications sepa- 

 rated by a deep groove, which is continued frora the mesial fold, with 

 one or two obtuse plications on either side, sometimes a single one 

 scarcely defined. On the ventral valve there is usually one plication 

 only in the sinus, which is margined by angular folds ; and outside of 

 these there is one, and rarely more, indistinct folds or undulations of 

 the surface. Rarely there are two plications in the sinus, one smaller 

 than the other, with three of unequal size on the mesial fold. 

 In well-preserved specimens, the surface is marked by fine elevated 



concentric striae and faint radiating strias. 



Of the interior, little is known beyond the fact that the dorsal valve 



is furnished with a distinct median septum, showing some peculiarities 



at the junction of the crura ; while the ventral valve shows short blunt 



dental lamellae. Should the entire interior characters become fully known, 



this will probably prove to belong to a distinct genus. 



