II 



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. 



Ihe 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 may be divided into several sections. Hisin- 

 ger makes two, — " StriataB and Lasvigatae ;" 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 " Spiriferae Tere- 

 bratuliformes " of Phillips ; the second, the remainder of 

 these, and his " Glabratae ;" and the third, the " FilosaB " 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 



c 2 



