350 
PALAEONTOLOGY OF NEW-YORK. 
Rliynchonella (Stenocisma) duplicate. 
PLATE LY. 
Atrypa duplicata : Hall, Report on Fourth Gaologicsl District of New-York ; Fossils of the Chemung 
group, Illustration lxvii, figs. 2 & 2 a. 1843. 
Shell depressed-suborbicular or subquadrate; 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 from 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 striae. 
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. 
