Brachiopoda of the Cincinnati Groiq). 23 



lateral extremities ; foramen very small and filled by tlie cardinal 

 process. Interior flat ; mesial ridge extending to about the middle of 

 the shell, without any well defined termination ; scars of posterior pair 

 of adductor muscles a little smaller than the anterior pair, from which 

 they are separated by a very fine line, or, more generally, not dis- 

 tinctly separated ; • cardinal process very small, conical, obscurely 

 trifid on the posterior side ; brachial processes slender, prominent, and 

 directed obliquely forward ; surface granular and showing the radiat- 

 ing strioe. 



Ventral valve convex, with elevated mesial ridge, greatest convex- 

 ity just behind the middle ; beak arched, projecting slightly ; area 

 moderate, narrowing laterally ; foramen an equilateral triangle, partly 

 occupied by the cardinal process of the other valve. Interior strongly 

 concave, showing moderately prominent teeth ; dental laminse extend 

 from the base of the teeth forw^ard,' gradually becoming more indis- 

 tinct as they fade away in a circular line to the mesial depression, 

 forming a heart-shaped cavity for the muscular scars ; surface granu- 

 lar and showing the radiating stride. 



Surface of both valves ornamented by fine radiating strice, that 

 increase by bifurcation ; lateral striae curved so that a few of them 

 run out on the hinge line ; concentric striae plainly visible with the 

 aid of a magnifier, and sometimes visible to the unaided eye ; imbri- 

 cating marks of growth usual. Length of an average full grown speci- 

 men, 0.50 inch ; breadth, 0.58 inch ; convexity, 0.20 inch. They vary, 

 however, from one fourth this size to one half larger. 



It is abundant at nearly all exposures from low water-mark, at Cin- 

 cinnati, to 250 feet above ; after this, as we ascend the strata, the form 

 which I have called Orthis Meeki prevails in its stead. It would be 

 impossible to determine where one form begins and the other ends, as 

 they clearly intermingle, and leave the constantly recurring impres- 

 sion that they are not specifically distinct. Take, however, either 

 young or old specimens from an elevation of 150 feet, and comj)are 

 them with like specimens in age from an elevation of 400 feet, and 

 the distinction is manifest, both externally and internally. It differs 

 from 0. Meeki, externally, as follows : It is smaller, more circular, 

 because of the shorter hinge line ; dorsal valve flatter and more usually 

 with a concentric indention. It is distinguished from 0. emacefata, 

 which hi associated with it in a limited range, both externally and 

 internally, as will be seen from the description of that species. I have 

 been unable to point out satisfactory differences between this form and 

 that described by Mr. James, as Orthis Cyclus, in this Journal, vol. 

 i., page 19, January, 1874, though jNIr. James may be correct in the 

 distinguishing characteristics he has designated. 



