FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 99 



This shell is described by Mr. Billings, of the Canadian Geological Survey, 

 under the name of Athyris clara, but, inasmuch as Mr. Conrad's name of 

 Atrypa nasuta preceded that of Mr. Billings, his species has priority, while 

 that of Billings must be dropped. 



Both in general exterior, form and internal characters, this species bears 

 much resemblance to Meristella (Atrypa) tumida of Dalman, but in Dalman's 

 species there is a distinct sinus in the ventral valve, while the umbo is more 

 gibbous, the beak larger and more incurved, and the entire shell is compara- 

 tively more ventricose. The muscular area in the ventral valve is of the same 

 shape, but narrower than the prevailing form in our species. 



Formation and Locality. Found in the upper strata of the Devonian group at and around the 

 Falls of the Ohio in Kentucky and Indiana. It is of rather rare occurrence, and fair specimens still more 

 so. 



Meristella unisulcata. CONRAD. 



Plate XV., figures 9 to 16. 



Atrypa unisulcata, Conrad. Ann. Rep. on the Pal. of N. Y., p. 56 1841. 

 Rhynchonella unisulcata (Conrad sp.), Hall. Tenth Rep. St. Cab., p. 125 1857. 

 Atrypa unisulcata, referred with doubt to Meristella, and name Goniocoelia proposed in Fourteenth Rep. 



St. Cab., p. 1011861. 



Compare: Pentagonia peersii, Cozzens. Ann. of N. Y. Lye., Vol. III., p. 158 1846. 

 Meristella unisulcata, Hall. Pal. N. Y., Vol. IV., p. 309, pi. L. 1867. 



Shell of medium size, sub-trigonal, quadrilateral or sometimes sub-pentago- 

 nal in outline; wider in front, with the sides sometimes sloping from the beak, 

 but in others the hinge-line is extended nearly straight, and the sides rectangu- 

 lar to it. A wide mesial depression on one side with prominent elevation. 



Ventral valve, with a broad, deep mesial sinus, which occupies nearly whole 

 width of valve, and is bounded on either side by an angular elevation, which 

 extends from beak to baso-lateral angles. The portion of the valve, outside of 

 limitation sinus, is abruptly inflected upwards or towards the dorsal valve, 

 often nearly at right-angles. Umbo is prominent, and beak is incurved over 

 umbo of dorsal valve. 



Dorsal valve gibbous in the middle ; the center occupied by a prominent 

 mesial fold, from which the surface slopes abruptly to the lateral angles, be- 

 coming more or less concave within the lateral and cardino-lateral margins. 

 The mesial fold is marked along the center by a single deep groove, which 

 extends to the beak of the valve. Surface marked by fine concentric striae, 

 and sometimes by strong imbricating folds ; there are also slight indications of 

 interrupted radiating striae. The specimens of our limestone are either exfoli- 

 ated or silicified; in both cases the finer markings are obliterated. The interior 

 of the ventral valve, specimens of which are found in an excellent state of pres- 



