74 TKENTON PERIOD. 



Orthis biforata Schlotheim, var. lynx. 

 Plato IV, fig. 9 a and i. 



Shell nearly equivalve ; width exceeding the length ; hinge-line vary- 

 ing in length from a little greater to a little less than the greatest width of 

 the shell, usually a .little less ; cardinal extremities generally more or less 

 proniinent in either case ; cardinal area present and nearly alike in both 

 valves ; they are narrow, well defined, each having a triangular foramen ; ' 

 dorso-ventral diameter varying with age from equal to one-half the trans- 

 verse diameter to the full extent of the same ; in the latter case, the shells 

 have a compact subglobose form. 



Dorsal valve having a prominent, well-defined mesial fold, sometimes 

 a little rounded but often quite angular, naiTow, but distinct at the beak, 

 and rapidly increasing in width toward the front; lateral portions convex 

 transversely, and regularly arching from front to rear; beak not prominent, 

 but arching over the area, and nearly meeting its fellow of the opposite 

 valve. 



Ventral valve about equally capacious with the dorsal; convexity of 

 its sides similar to that of the sides of the dorsal valve, having a deep 

 sinus corresponding with the elevated mesial fold of the other valve; its 

 beak also similar in size, prominence, and incurvatvu-e to that of the dorsal 

 valve. 



The surface of each valve is marked by from eighteen to twenty-six 

 strong, usually angular, plications, either three or four of which are at the 

 bottom of the mesial sinus, and either four or five of them upon the mesial 

 fold. The plications at each side of the mesial fold and sinus are similar 

 in character to the others ; all being usually simple, or continuous from the 

 beak to the margin. 



Length, about twenty-five millimeters; breadth, about thu-ty-three 

 millimeters. 



Several varieties of this exceedingly variable species, both in this country 

 and Europe, have been described as distinct species. Its geographical dis- 

 tribution being very great, and notices and descriptions of it having been 

 published at various times and places during more than one hundred and 

 fifty years, its synonymy has attained unusual magnitude and diversity. 



