2S6 PAL.EOXTOLOGY OF OHIO. 



the breadth, and the breadth usually about twice the height of the area ; 

 hinge line about equaling the greatest breadth ; front and lateral mar- 

 gins forming together a more or less nearly semicircular curve, or with 

 the central part of the former sometimes a little straightened, or even 

 ver}' faintly sinuous in outline, and the latter meeting the hinge at rather 

 less than right angles behind. Dorsal valve moderately convex in the 

 central region, thence sloping laterally, and rounding more abruptly 

 to the beak and anterior lateral margins than to the middle of the front ; 

 mesial fold depressed, smoothly rounded, equaling about two-thirds the 

 breadth of the valves at the front, and" sometimes showing on internal 

 casts a faint linear mesial impression; beak small, and with the very 

 narrow area incurved. Ventral valve much elevated at the beak, thence 

 sloping laterally, with slightly convex outlines, at an angle of 100° to 125°, 

 and more abruptly to the front and anterior lateral margins ; mesial 

 sinus smoothly rounded within, rather shallow, or moderately deep an- 

 teriorly, where it terminates in a short rounded projection fitting into a 

 corresponding sinus in the margin of the other valve; beak elevated, 

 obtusely angular and straight, or a little arched backwards ; area high, 

 transversely and vertically striated, ranging more or less nearly at right 

 angles to the plane of the valves, and flattened or somewhat arched 

 backward, with its lateral margins moderately well defined ; foramen 

 large, or about two-sevenths as wide at the hinge line as the length of 

 the latter, and three-fifths as wide as high, showing its deep-seated 

 transverse septum and tube to be well developed above within. 



Surface of both valves ornamented on each side of the non-costate 

 mesial fold and sinus by about eighteen to twenty simple, depressed, 

 rounded, radiating costse, some five or six of which, on each of the lateral 

 extremities of both valves, are usually nearly or quite obsolete.* Cross- 

 ing all of these, on well-preserved specimens, numerous fine concentric 

 strife and some stronger marks of growth may be seen; and over the 

 whole a minute pitting may be observed, so crowded and arranged as to 

 present a delicate textile appearance, as seen by the aid of a magnifier. 



Breadth of a well-developed, mature specimen, 2.70 inches; length, 

 about 1.37 inches; length of hinge line, 2.65 inches; height of area, 1.25 

 inches ; breadth of foramen at hinge, about 0.73 inch. 



This shell seems to agree almost as well with Spirifer textus, Hall, from 

 the fine-grained (Carboniferous) sandstone of the Knobs, near Louis- 



* The costae are too sharply defined, and represented too small and too numerous 

 on the ventral valve in our figures 7a and 76. They are about right on the dorsal 

 valve in fig. 7a; but the lateral slopes of the ventral valve in the same figure are in- 

 correctly drawn straight, instead of somewhat convex, thus making the lateral ex- 

 tremities of the figure too acutely angular. 



