N 
ATRYPA, Dalman. 
Gen. Char. Shell equilateral,, unequal-valved, with 
both valves convex ; hinge furnished with two 
spiral appendages, its margin short, arched ; the 
area between the beaks small, generally convex, 
rarely flat, with a triangular pit or notch in the 
middle, closed by a deltidium admitting the 
smaller beak ; beaks pointed, incurved ; animal 
a Brachiopodous Mollusk. 
The shells to which Dalman has given the generic name 
Atrypa, because they have no circular aperture at the ex- 
tremity of the larger beak, although in general appearance 
they resemble Terebratulae, were placed by the late Mr. 
Sowerby in his genus Spirifer. They form a very natural 
genus, which maybe divided into several sections. Hisin- 
ger makes two , — iC Striatae and Laevigatas we would 
prefer dividing the latter into two, and shall then have 
three sections, viz., 
1st. Not striated longitudinally, and with the edges of 
the valves on one plane. (Cardinal area none.) 
2nd. Not striated longitudinally, and with the front of 
the larger valve elevated. 
3rd. Striated or plaited longitudinally. 
The first section contains some of the u Spiriferae Tere- 
bratuliformes ” of Phillips ; the second, the remainder of 
these, and his u Glabrataa and the third, the “ Filosae ” of 
the same author. Several species of the two first sections 
are ornamented with concentric laminae or fringes, and 
among the latter are some with imbricating scales on the 
surface. Most of the species have two longitudinal septa 
in the beak of the larger valve and one in the other ; these 
are very broad in some of the species belonging to the se- 
cond section (by which circumstance they approach the 
genus Pentamerus) ; in others they are reduced to mere 
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