Welter — Kinderhook Fanned Studies. 99 



igan collection, and which also agrees with Winchell's de- 

 scription and dimensions. Associated with this specimen, 

 however, and attached to the same card, are other specimens 

 which seem to be distinct. Winchell recognized this other 

 form and mentioned it in his description, but was not inclined 

 to consider it as specifically distinct from the type. These 

 two shells, however, seem to be entirely distinct, and in the 

 material examined there are no intermediate connecting forms, 

 so that the second form will be described here as a distinct 

 species. 



Sphenotus bicostatus n. sp. 



Pl.IV.f. s. 



Shell subelliptical in outline, strongly convex, length, 29 

 mm., width, 14 mm. Hinge-line arcuate; anterior margin 

 curving abruptly inward to the anterior extremity of the 

 hinge-line under the beak, more gently and regularly curved 

 below ; ventral margin straight or gently curved through the 

 greater part of its length but more strongly curved upward 

 both anteriorly and posteriorly ; posterior margin sharply 

 rounded below, obliquely truncated above. Beak elevated 

 above the hinge-line, flattened, incurved, directed forward, 

 situated about one-fourth the length of the shell from the an- 

 terior end. A prominent, rounded, umbonal ridge extends 

 from the beak to the postero-ventral margin ; the postero- 

 dorsal slope is marked by two additional subangular ridges 

 radiating from the beak, one just at the dorsal margin and 

 another midway between the dorsal margin and the umbonal 

 ridge. The dorsal margin is sharply inflected to form a 

 groove for the attachment of the ligament, and the area be- 

 tween the dorsal marginal ridge and the median one is con- 

 cave. The greatest convexity is near the middle of the valve 

 on or just below the umbonal ridge. A shallow, ill-defined 

 sinus extends from the beak obliquely backward to a point 

 anterior to the middle of the ventral margin. Surface marked 

 by fine concentric lines of growth. 



Remarks. This shell was included by Winchell in his 

 species 8. iowensis, the specimen here illustrated being at- 

 tached to the same card with the type specimen of that species. 



