TRENTON LIMESTONE. 139 



Straight hinge line, and produced front of the medial lobe, as far as can be ascertained, 

 though it has some obscure longitudinal striae. 



The shell is frequently much expanded laterally, and compressed towards the edges ; 

 the beaks are almost equally elevated, that of the ventral valve being more gibbous than 

 the other. There is some variation in the sinus of the dorsal valve, which, in some specimens, 

 is more produced and acute in front than in others, while the medial lobe often becomes 

 angular. 



This species is apparently quite distinct from A. nucleus, which has a flat sinus in the 

 dorsal valve, rounded at its extremity, and the surface of the shell presents ho longitudinal 

 striae. 



Fig. 1 a. Ventral valve of a perfect specimen, b. Front view of the same. 



Fig. 1 c. Dorsal valve, d. Profile view. 



Fig. I e. Front view of a specimen, where the sinus is narrower and more angular than in the preceding. 



Fig. I / Profile view of the sama 



Fig. 1 g. Dorsal valve of a specimen, which is more extended laterally. 



Fig. 1 k. Front view of the same specimen. 



Position and locality. This species is known to me only as occurring in the central part 

 of the Trenton limestone, at Lowville, Lewis county. (State Collection.) 



176. 9. ATRYPA BISULCATA. 



Pu XXXIII. Figs. 3 a, b, c, d, e. 

 Orthu bituleata. Emmons, Geol. Report, 1842, p. 395, fig. 4 a, b. 



Small, ovoid ; dorsal valve with a well defined, narrow, mesial sinus, which continues 

 about half way to the beak, and from there the centre becomes much elevated ; beak of the 

 dorsal valve strongly incurved over that of the opposite valve ; ventral valve depressed- 

 convex, prominent on the umbo, beak very small and abruptly incurved ; front with two 

 short well defined furrows, ending in two plications, which close on each side of the 

 projecting plait formed by the extension of the mesial groove of tlie dorsal valve. 



This peculiar little shell has no analogue in any of the lower strata, though there are 

 one or two approaching to it in form in the higher rocks. Its neat ovoid form, smooth or 

 concentrically striated surface, with the two small plaits and grooves in one valve and one 

 in the other, are marks to be sought for in identifying the species. It is extremely rare, and 

 has been found only in one or two localities. 



Fig. 3 a. Dorsal valve, b. Ventral valve, c. Cardinal view. e. Front view. d. Enlarged profile view. 



Position and locaiity. Adams, Jefferson county, in shaly Trenton limestone, associated 

 with MuRCHisoNi.\ and Pleurotomaria, and in a situation where few Brachiopods occur. 



{State Collection.) 



18* 



