Bvachiopoda of (lie Cincinnati Group. 37 



It is first found iu the hills back of Cinciunati, at an elevation of 

 about 320 feet, and continues to the top of the hills. It is found at 

 Versailles, and Weisberg, Indiana, in Butler, Warren, and Clermont 

 counties, Ohio, and generally wherever the rocks are well exposed, 

 from an elevation of 320 feet, at Cincinnati, to the top of the rocks of 

 the Cincinnati Group. Though good specimens can hardly be said to 

 be abundant. 



OHhis retrorsa — (Salter, 1858). 



■ Shell attaining a large size, varying from transversely suboval to 

 truncato-suboval, or subquadrate, the length being about four fifths its 

 breadth ; hinge line shorter than the greatest breadth of the valves, 

 or about equaling their length, with its lateral extremities abruptly 

 rounded or very obtusely subangular; lateral margins more or less 

 convex in outline, and rounding to the front, which is either regularly 

 rounded or somewhat straightened along the middle ; valves decidedly 

 unequal, or concavo-convex. 



Dorsal valve evenly and sometimes rather distinctly convex, the 

 most prominent part being near, or a little behind the middle, with 

 the anterior and lateral slopes, particularly the anterior, more gradual 

 than the posterior ; umbonal convexity projecting a little beyond the 

 hinge ; beak incurved ; area of moderate height, but with its sharp 

 margins sloping oft" to nothing near the extremities of the hinge, more 

 or less strongly incurved, so as sometimes to stand at right angles to 

 the plane of the valves ; foramen broad triangular. Interior imknown. 



Ventral valve convex at the point of the beak, and thence sloping 

 toward the lateral and anterior margins; the anterior central, and 

 sometimes the lateral regions, being more or less concave ; beak obtuse, 

 or abruptly pointed, and strongly incluaed forward; cardinal area 

 broad, triangular, well defined, flat, and so distmctly inclined forward 

 as to place the apex of the beak some distance in front of the hinge 

 margin ; foramen narrow-triangular, being often a little higher than 

 wide, and extending to the apex of the beak. 



Interior with muscular cavity rather deep, distinctly quadrangular, 

 scarcely reaching the middle of the valve, and sharply defined by a 

 raised margin, which is perfectly straight and uninterrupted across the 

 front ; while its lateral margins are each waved a little outward, along the 

 middle, to make room, as it were, for the scars of the ventral adjuster 

 muscles, which are moderately distinct from those of the longer trian. 

 gular divaricators ; scars of the adductor muscles well defined, and 



