Wetter — Kinderhook Faunal Studies. 75 



valve without an appreciable mesial fold. Surface of each 

 valve marked by twenty or twenty-two prominent, simple, 

 rounded plications, which gradually increase in size towards 

 the front; the furrows between the ribs much narrower than 

 the ribs themselves ; the groove along the median line of the 

 pedicle valve stronger than any of those on either side. 

 Length of an average specimen 19 mm., breadth, 15 mm., and 

 thickness, 10 mm. 



Remarks. The types of this species in the University of 

 Michigan collection are four in number, all of them more or 

 less imperfect. The specimen here illustrated is a more 

 perfect one in the University of Chicago collection from 

 the same locality as the types. The species resembles 

 E. marcyi (Shum.) from the St. Louis limestone fauna, 

 but it has a more acute and more erect beak, coarser plica- 

 tions, and the slight median sinus with its stronger median 

 furrow. 



Athyris corpulenta (Win.). 



Pi. II. f. 12-15. 



Shell varying from longitudinally subelliptical to subcircular 

 in outline, the valves becoming exceedingly gibbous in the 

 adult individuals. Pedicle valve gibbous in the middle, 

 usually with a shallow, ill-defined mesial sinus in the anterior 

 half of large shells, though this is sometimes practically 

 obsolete; in those specimens with the sinus well developed, 

 the anterior margin of the valve is produced; the beak is 

 small and incurved over that of the brachial valve. Brachial 

 valve nearly as convex as the pedicle valve so that the older 

 and shorter individuals become subglobular in form; the 

 specimens with a sinus developed in the pedicle valve have a 

 corresponding, ill-defined fold in the brachial valve. Surface 

 of both valves marked by strong, concentric lines of growth 

 which are often crowded near the margin. The dimensions 

 of a small, exceedingly elongate specimen are : length, 15 mm., 

 breadth, 11 mm., and thickness, 10 mm.; those of another 

 larger and very gibbous specimen are: length, 16 mm., 

 breadth, 18 mm., thickness, 20 mm. This latter specimen has 

 apparantly been somewhat distorted so that the convexity 



