Brachiopoda of the Cincinnati Group. 43 



is situated on cither side at the base of the fissure, supported by a den- 

 tal ridge or plate ; these diminish in size as they converge under the 

 area toward the extremity of the beak. At the bottom of the valve, 

 under the beak, and extending a little beyond, are the impressions left 

 by the cardinal and adductor muscles, which occupy about one third of 

 the length of the valve ; they form two elongated oval scars, more or 

 less deeply excavated, and separated by a rather wide mesial ridge. 

 In the interior of the smaller or dorsal valve the cardinal process is 

 largely developed, being composed of two testaceous projections, which 

 are either slightly convex or concave on the side facing the interior, 

 but grooved or bidentated toward the extremity of their outward sur- 

 face ; the socket plates are large and partly united to the lower portion 

 of the cardinal process. Under these, on the bottom of the valve, are 

 seen the quadruple impressions left by the adductor, which occupy 

 more than a third of the length of the valve, and are arranged in 

 pairs, divided by a short, rounded mesial ridge." 



The species found in the Cincinnati exposure of the Lower Silurian 

 belong to a sub-group having the following characters : 



Beak of neither valve prolonged, but, on the contrary, projecting so 

 slightly, as to be scarcely distinct from the edge of the area ; hinge line 

 usually as long or longer than the width of the shell further forward ; 

 lateral extremities usually rectangular or terminating in regular points 

 more or less deflected ; beak of ventral valve perforated. 



Interior of dorsal A'^alve with bifid cardinal process ; more or less 

 distinct mesial ridge, and longitudinal furrows ; socket ridges short 

 and oblique. Interior of ventral valve with hinge teeth, divergent, 

 oblique, and moderately developed ; dental ridges encircling or nearly 

 encircling a more or less saucer-shaped cavity, in which are situated 

 the muscular scars, separated by a more or less defined mesial ridge. 



I have followed Prof. Meek in using Hemipronites instead of Strep- 

 torhynchus, though in the Ohio Paleontology he has, as I think, erron- 

 eously called it a sub-genus. Prof. Hall uses the name Streptorhynchus 

 for all these shells. 



Hemipronites filitexta — (Hall, 1847). 



Shell semi-oval, length usually less than the breath ; hinge line gen- 

 erally greater than the breadth at any point farther forward ; lateral 

 extremities usually somewhat acutely angular, and deflected ; lateral 

 margins a little sinuous posteriorly, and rounding to the semi-circular 

 front. 



Dorsal valve flat in the umbonal region, and evenly convex in the 



