FOSSILS OF THE CLIXTON GKOUP. 115 



BRACHIOPODA. 



Genus STROPIIOMEISrA, Ralinesque. 

 Strophomena patenta. 



Plate 5, fig. 10. 



Lepfxna patenta, Hall; Pal. N. Y., Vol. IL, p. 60, pi. 21, fig. 36. 

 Strophomena paterda, Hall; 12th Eept. State Cab., p. 82. 



Shell of medium size, semi-ovate in outline, wider than long ; valves 

 resupinate, the ventral being concave beyond the middle of the shell. 

 Hinge line as long, or longer, than the width of the shell below ; front 

 and sides of the shell rounded, the latter usually constricted just below 

 the hinge line, forming ear-like extensions at the cardinal extremities. 

 Width of the cardinal area not determined. Ventral side convex on the 

 umbone and central region of the valve, but strongly recurved beyond 

 the middle, forming a deeply concave shell in the more advanced stages 

 of growth. Dorsal valve flat, or slightly concave, on the umbone, and 

 strongly geniculate in front, conforming in curvature to that of the oppo- 

 site valve. Surface marked by fine, closely arranged,' bifurcating or im- 

 planted radiating striae, which become somewhat coarser with the in- 

 creased size of the shell ; also by very fine, concentric, undulating striae, 

 often very distinct between the radiating lines, forming a beautifully 

 cancellated surface when well preserved, and by several more distinct 

 concentric lines, indicating stages of growth. 



The species belongs to a groujj of Strophomenoid shells which are 

 characterized by having the ventral valve convex in the young shells, 

 while in the more advanced stages, or adult individuals, it becomes 

 doubly flexed, and often deeply concave. The shells have much the 

 form and character of those Silurian species referred to the genus Strep- 

 torhynchus, but difier from them very strongly in this particular of having 

 a double curvature to the shell; and this feature will serve to distinguish 

 this species readily from Streptorhynchus subjjlana, with which it is asso- 

 ciated. The shells under consideration appear to be identical with those 

 from New York, above cited, differing onl}'' in the somewhat finer radiat- 

 ing stria3 of the surface. This difference, however, may be more ap- 

 parent than real, as the New York examples are thoroughly silicified 

 and the surface features somewhat obscured by this change. 



Formation and locality : In the limestones of the Clinton group, at Dayton, Ohio. 



