26 Bracliiopoda of the Cincinnati Group. 



extremities obtusely angular, rectangular, or acutely angular ; lateral 

 margins convex, nearly straight or sinuous behind and rounding to the 

 front, which is sinuous, rounded, or somewhatprominentat the middle; 

 beaks and cardinal areas of the two valves nearly equal, the former 

 incurved and approximate, or sometimes contiguous. 



Dorsal valve most convex, greatest convexity near the middle, 

 mesial fold rounded, moderately prominent, commencing near the 

 beak, and continuing forward, gradually widening and rounding with 

 the curve of the valve to the front ; lateral slopes convex ; beak pro- 

 jecting beyond the hinge margin, strongly incurved ; cardinal area 

 well developed, nearly as wide as that of the other valve, directed 

 backward, and more or less incurved ; foramen broad, triangular, and 

 not closed by the cardinal process. Interior showing cardinal process 

 to be very small, or merely having the character of a low, linear ridge in 

 the rostral cavity ; posterior pair of muscular impressions corrugated, 

 and decidedly larger, and more widely separated than the others. 



Ventral valve with a mesial sinus corresponding to the fold in the 

 other valve, and terminating at the front in a rather short, somewhat 

 rounded projection, that curves more or less upward into a sinuosity 

 of the same size and form in the margin of the dorsal valve ; beak 

 less strongly incurved than that of the other, and slightly more promi- 

 nent at its apex ; cardinal area one fourth to one third higher at the 

 middle than that of the dorsal valve, and narrowing less rapidly 

 toward the lateral extremities ; beak incurved, projecting beyond the 

 margin ; foramen approaching in form an equilateral triangle. 

 Interior witli hinge teeth moderately prominent and trigonal ; 

 muscular cavity comparatively small, elongate-oval, sides nearly 

 parallel, scarcely reaching the middle of the valve, well defined by 

 the dental ridges, and sometimes extremely profound. 



Surface of each valve ornamented by 16 to 24 strong, more or less 

 angular, radiating plications, of which 3 to 5 occupy the mesial sinus, 

 and 4 to 6 the mesial fold ; plica'tions usually simple, but sometimes 

 bifurcate once ; lines of growth near the free margins present a zigzag 

 ap|)earance in crossing the plications and furrows ; surface sometimes 

 shoAving under a magnifier minute granules. 



Shells regarded as belonging to this species are found at Cincinnati, 

 within 250 feet of low water-mark, and from thence extend through- 

 out the Cincinnati Group. Large specimens are rare until an 

 elevation is reached of about 400 feet, and the earlier forms usually 

 have a length and breadth greater than their convexity. At 425 feet 

 a bed is reached, about ten feet in thickness, containing the largest of 

 specimens ; these usually have a convexity about equal to their length. 



