86 PAL^ONTOLCGY. 



extremities more or less acutely angular ; valves nearly equally convex, the 

 dorsal being generally most convex in front, and the ventral in the umbonal 

 region. Dorsal valve with its beak not very prominent or strongly incurved 

 in the type-specimens, in which the area is rather nan-ow, well defined, nar- 

 rowed to the lateral extremities, and a little arched and inclined backward 

 over the hinge ; foramen triangular, slightly higher than wide, and provided 

 with a distinct marginal fuiTow on each side ; mesial sinus rather narrow, 

 well defined, smoothly rounded within, and commencing at the apex of the 

 beak, from which it widens and deepens gradually to the front ; lateral 

 slopes on each side of the sinus ornamented by from nine to twelve or thir- 

 teen simple, prominent, rather acutely-rounded costse. Ventral valve with 

 a narrow, well-developed cardinal area, which generally tapers to the hinge- 

 line before reaching the lateral extremities, and, with the short beak, arches 

 rather strongly inward ; mesial elevation simple, narrow, rounded, and con- 

 tinued to the beak ; lateral slopes with their simple costse corresponding in 

 number and arrangement to those of the other valve. Surface of both 

 valves with small, somewhat obscure, subimbricating lines of gi-owth, which 

 are rather distinctly arched in crossing the costse, fold and sinus, show- 

 ing the punctate structure very clearly under a common single pocket- 

 magnifier. 



Length (of one of the more compressed specimens), 0.95 inch ; breadth 

 of the same, about 1.76 inches ; convexity, 0.52 inch. 



Most of the typical specimens of this species are rather compressed, 

 and have the lateral extremities extended and acutely pointed. Other 

 specimens, however, from the same rock at localities a little farther north, 

 found associated with the same group of fossils, are more gibbous, and have 

 slightly smaller and a little more crowded costse, with a somewhat higher 

 and more arched area and beak. As the latter, however, vary somewhat 

 in these respects, and agree exactly in all other known characters with the 

 more compressed typical examples, I have not been willing to separate 

 them. 



Although not unlike some of the described species of Spirifer (Tri- 

 gonotreta), I am not acquainted with any described species of Spiriferina so 

 near this as to render a comparison necessary. It certainly belongs to 



