OrtJiis cyclns — James. 19 



Descripthns of Xew Species of Brachiopoda, from tJie Loicer Silurian 

 Bods — Cincinnati Group ; by U. P. James. 



Orthis cyclvs. — (James.) 



[^Cijclus, in allusion to the circular form of the shell.] 



►Shell small, much compressed, sub-circular, broader than long ; 

 length, about six sevenths of the breadth ; cardinal line about two 

 thirds as long as the greatest breadth of the shell ; margins rounding 

 from the extremities of the cardinal line to the front, presenting an 

 outline of more than two thirds of a circle ; slightly sinuous in the 

 front ; valves very thin and closely drawn together for more than two 

 thirds the surface area. 



Dorsal valve convex above the middle, but nearly flat toward the 

 free margins, with a slight mesial sinus from near the beak extending 

 toward the front. Beak very small, slightly projecting, not incurved ; 

 hinge line straight and narrow, short ; foramen very small, filled by 

 the cardinal process of the other valve. Interior shallow, with quite 

 a strong mesial ridge extending, in some cases, to the front, in others, 

 flattening out before reaching the front margin. No definite muscular 

 impressions below the teeth ; internal surface showing the striae of the 

 exterior of the shell distinctly. 



Ventral valve nearly flat, more than two thirds of the surface 

 area, convex on the umbone, quite abrupt, with a slight mesial ridge 

 flattening out toward the front margin. Beak projecting beyond the 

 dorsal valve, incurved ; area much higher than the dorsal valve, 

 sloping rapidly toward the extremities of the hinge line. Interior: 

 teeth prominent ; lateral muscular cavities, under the hinge line, at 

 the sides of the teeth, deep ; muscular scars, none ; quite a deep 

 mesial sinus extending from the cardinal process about half way to 

 the front margin. External striae distinctl}' shown through the shell, 

 as in the dorsal valve. 



Surface of valves covered with prominent, rather coarse radia- 

 ting striie, which bifurcate before reaching the free margins, and are 

 strongly curved back along the hinge line. Numerous very fine 

 concentric lines between and across the striae ; with, occasionally, 

 strong imbricating lines of growth in some specimens. 



Length of mature, rather large, individuals, 0.42 inch ; breadth, 

 0.50 inch. 



This shell resembles 0. emacerata, Hall, but differs in the coarser 

 striae and internal markings, and is scarcely half the size of mature 

 specimens of that species. It also bears some resemblance to 0. mul- 



