Weller — Kinderhook Faunal Studies. 91 



hinge-line, but little incurved. Umbonal ridge broadly 

 rounded, ventricose, the postero-dorsal slope concave, forming 

 an alate expansion of the shell. Surface marked by fine, 

 regular, sharp, concentric lines of growth. 



Remarks. This shell is represented by a single specimen, 

 the type, whose posterior portion is imperfectly preserved, 

 this part being restored in outline in the illustration, as indi- 

 cated by the direction of the lines of growth. As originally 

 described it was referred to the genus Area, but it should 

 rather be referred, in all probability, to the genus Macrodon. 

 It differs from the species of this latter genus in general, in 

 its broadly rounded ventral margin, and its great breadth 

 posteriorly as compared with the anterior portion of the shell. 

 The true generic relationship of the species can of course be 

 determined only when its hinge-structure can be known. 



Grammysia plena Hall. 



Pi. iv. f. 21. 



Shell subovate or subelliptical in outline, length, 36 mm. 

 and height, 22 mm. Hinge-line straight, more than half the 

 length of the shell; anterior margin short, abruptly rounded 

 below the lunule which is deep and distinct extending half 

 way or more from the beak to the base of the shell ; ventral 

 margin regularly curved from end to end except for a slight 

 constriction at about the anterior third ; posterior margin 

 rounded below, obliquely truncate above. Valves regularly 

 convex in the posterior portion, becoming very gibbous in the 

 middle and umbonal region. The beaks prominent, much 

 elevated above the hinge-line, strongly incurved, situated 

 near the anterior extremity ; umbonal slope rounded or sub- 

 angular, the postero-dorsal slope sometimes marked by a dis- 

 stinct fold along the middle ; a shallow sinus extends from 

 the beak obliquely backward to the constriction in the ventral 

 margin. Surface marked by fine concentric striae which 

 become fasciculate posteriorly, by fine pustulose striae which 

 can only be seen in exceptionally well preserved specimens, 

 and by strong, subangular or rounded concentric undulations 

 which are strongest anterior to the umbonal ridge, being re- 

 placed posteriorly by the fascicles of fine concentric striae. 



