Welter — Kviderhook Faunal Studies. 155 



erately gibbous, about as long as wide» or sometimes slightly 

 wider than long ; greatest breadth near the middle ; posterior 

 lateral slopes rather straight, and converging to the beaks at 

 an angle of about 100 degrees; sides more or less rounded or 

 sometimes subtruncate. Pedicle valve depressed-convex in 

 the umbonal and lateral regions, and concave in the middle, 

 the concavity commencing narrow and shallow, generally be- 

 hind the middle, and widening and deepening to the front, so 

 as to form a broad, shallow, rather flat mesial sinus; de- 

 pressed part of the front curving downwards, and a little 

 produced, to fill a corresponding sinuosity in the front of the 

 other valve, the margins of the two valves meeting there, at 

 rather less than a right angle, so that no emargination of the 

 outline of the front is produced; beak small, rather pointed, 

 projecting little beyond that of the other valve, over which 

 it curves. Brachial valve considerably more convex than the 

 other, the greatest convexity being generally in front of the 

 middle, from which it rounds off abruptly behind and on 

 each side, while in the middle it rises into a broad depressed, 

 or moderately prominent, flattened or somewhat rounded, 

 mesial prominence, rarely extending back much beyond the 

 middle; beak incurved; cardinal margin broadly and rather 

 distinctly sinuous on each side of the beak. 



*' Surface ornamented by about nine to eleven broad, dis- 

 tinct, rounded, occasionally bifurcating plications, most of 

 which, excepting the outer lateral ones, extend nearly to the 

 umbones. Of these plications, three to four occupy the 

 mesial sinus and four to five the mesial fold, the greater num- 

 ber in each instance generally resulting from the bifurcation 

 of one of the lateral ones. Distinct, rather coarse, irregular 

 radiating striae also mark every part of the surface, and are 

 well defined on exfoliated surfaces, as well as upon internal 

 casts, while fine undulating lines, and occasional stronger 

 marks of growth, traverse the surface concentrically. 



*' Length of a mature specimen 24 mm. ; breadth 25 mm. ; 

 convexity 17^ mm.; also of another more gibbous indi- 

 vidual, of the same size, 19 mm.'* 



Remarks. The specimens of this species from bed No. 4 

 agree exactly with the typical form of the species as it was 



