Conlributions to Iiwertehrale Paleeonlologij. 585 



in the prominence of the mesial fold ; but the form of the plications 

 and the character of those marking the fold and sinus are usually the 

 same in all ; while the most constant and persistent character, and 

 one I have been able to detect on specimens from almost every 

 locality noticed, consists of the minute structure of the surface. I 

 have lately examined a large number of examples from the lime- 

 stones and sandstones of the Coal Measures of New Mexico, which 

 correspond exactly with those figured by Prof. Marcou under the 

 name S. Rockymontana, and find them showing all the variations 

 in form noticed among the Keokuk, St. Louis, Chester, and Coal 

 Measure limestones of Ohio and the West, and am thoroughly con- 

 vinced they cannot be separated, even as local varieties, Avith any 

 degree of safety or satisfaction. 



Formation and Locality. — The specimen figured is from the 

 Maxville limestone (Chester), at Newtonville, Ohio. 



Genus ATHYRIS McCoy. 



Athyris subqiiadrata. 



Plate XIV, figs. 1-3. 



Athyris subqiiadrata Hall, Geol. Iowa, vol. i, pt. 2, p. 703, pi. 27, fig. 2, and 

 p. 708, fig. 118. 



Shell small or of medium size, subqnadrate in outline and strongly trilo- 

 bate, very variable in its proportional length and breadth, varying from longer 

 than wide to much wider than long. Valves ventricos«, the ventral the most 

 rotund, with the beak more or less prolonged and incurved, the extremity dis- 

 tinctly and rather strongly truncated and perforated by a round foramen of 

 considerable size ; the middle of the valve is marked by a rather deep, more 

 or less angular mesial sinus, which extends to the beak, but is faintly marked 

 in the upper third of its length, becoming strong and distinct toward the front 

 where the shell is prolonged and bent upward in a lingulate extension. Dorsal 

 valve most rotund on the umbo, the beak obtuse and incurved ; middle of the 

 valve strongly elevated in front, forming an abrupt, rounded, mesial fold, 

 which is not strongly marked posterior to the middle of the length, and scarcely 

 defined in the upper part. On the sides, the shell is bent downward, forming 

 on each side of the fold a deep sulcus, outside of which the shell is again 

 inflated or elevated, giving a strongly trilobed form to the front half of the 

 valve. Surface of the shell marked only by concentric lines of growth, whicli 

 are mostly confined to the anterior portion, and are often very numerous and 

 crowded, giving the shell a much thickened appearance on the margin. 



The species is a well-known type of the Chester limestones of 

 Illinois and Kentucky, and is often identified with AthyrHs ( Tere- 

 Annals N. Y. Acad. Sci., V, Jan. 1891.— 39 



