36 Brachiopoda of the Citicinnati Groiip. 



striated cavity within the beak very small, and broad triangular, free 

 margin crenate within, while the surface between these and the deeply 

 impressed muscular cavity is usually smooth, or sometimes very 

 minutely and obscurely corrugated ; vascular markings unknown. 



Surface of both valves ornamented by distinct, rather prominent, 

 radiating strise, which on the dorsal valve nearly always increase by 

 intercalation, and curved gradually outward on the posterior lateral 

 regions ; while on the ventral valve they generally increase by bifur- 

 cation, and are nearly straight on all parts. A few distant imbricat- 

 ing marks of growth are also usually seen around the free margins of 

 adult examples, while well preserved specimens show minute but not 

 crowded, prominent, concentric lines crossing the much larger radiating 

 strise, and the furrows between them. 



It is found from about 300 feet above low water-mark, at 

 Cincinnati, to the Upper Silurian rocks ; becoming most abundant in 

 the last 100 feet of the upper part of the Cincinnati Group. 



OrtJiis sinuata — (Hall, 1847). 



Semi-oval, with a sinus in front ; cardinal line scarcely equal to the 

 width of the shell ; dorsal area large, triangular ; foramen triangular, 

 reaching to the beak, the upper margins sloping rather abruptly from 

 the beak; ventral area narrow, linear; foramen broad, triangular, with 

 a distinct medial tooth, which reaches as high as the area ; dorsal valve 

 convex, its greatest elevation at the point of the beak, which is acute ; 

 a depression along the center, which becomes a sinus in older 

 shells ; ventral valve regularly convex in young specimens, gibbous, 

 somewhat emarginate and elevated in front in older specimens ; surface 

 marked by strong, regular rounded stride, which bifurcate in a nearly 

 uniform manner about half way to the base ; striae crossed by elevated, 

 subimbricating, concentric lines. A few imbricating lines of growth 

 are distinct toward the margin of the older shells. 



It is distinguished from the 0. occidentalis by the stronger and more 

 prominent strice, which more regularly bifurcate. The beak of the 

 dorsal valve is more elevated and acute, giving a greater height to the 

 dorsal area. The ventral valve is about equally convex or gibbous, 

 but it never exhibits any depression along the center. The depression 

 or sinus in the dorsal valve is usually more abrupt, deeper, and often 

 accomipauied by a corresponding elevation on the ventral valve, which 

 does not occur in the 0. occidentalis. The young shells are more 

 gibbous and have the beak of the dorsal valve more elevated and 

 acute. 



