472 GEOLOGY OF OHIO. 



form and number of the plications, and the minute surface structure 

 being exactly as in those. 



The form of the shell will vary from longer than wide to much 

 wider than long, dependent on the extension of the hinge-line. In pro- 

 file the shell is extremely ventricose, with a strongly enrolled beak ; a 

 moderate cardinal area, vertically striated ; a well-marked mesial fold and 

 sinus ; from seven to ten simple, rounded, or sub-angular plications on 

 each side, and from four to six bifurcating or dividing plications on the 

 fold and sinus. The plications and intervening spaces, when the surface 

 is well preserved, are marked by fine longitudinal lines, showing even on 

 partially exfoliated specimens, and are also crossed by still finer trans- 

 verse striae which undulate in crossing the plications, and on perfectly 

 preserved surfaces appear to be minutely setose on their edges. 



The species is extremely variable in its general outline, as exhibited 

 among the collections from all of the many localities from which I have 

 examined specimens, especially in the extension ot the hinge-line, and 

 the proportional width of the shell below, and also 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 have been able to de- 

 tect 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 limestones and sandstones of the Coal Measures of 

 New Mexico, which correspond exactly with those figured by Prof. Mar- 

 cou 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 the} r cannot be separated, even as local varieties, with any degree 

 of safety or satisfaction. 



Formation and Locality. — The specimen figured is from the Maxville 

 limestone (Chester), at Newton ville, Ohio. 



Genus ATHYRIS McCoy. 



At h wis subquadrata. 



Plate X, figs. 1-3. 



Athyris subquadrata Hall, Geol. Iowa, vol. i, pt. 2, p. 703, pi. 27, fig. 2, and p. 708, 

 fig. 118. 



Shell small or of medium size, subquadrate in outline and strongly trilobate, 

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

 to much wider than long. Valves ventricose, the ventral the most rotund, with the 

 beak more or less prolonged and incurved, the extremity distinctly 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 



