liELDERBERG FORMATION IN ^MARYLAND 35 



of the ventral valve is straighter and not so closely incurved over that of 

 the dorsal valve. It has in some individuals more plications and the 

 plications are more angular than in Uncinuhis nucleolaius. In the an- 

 terior portion the sinus and fold when present are less distinct and the fold 

 is not prolonged anteriorly to form the linguiform extension seen in 

 Uncinuhis nucleolaius. 



Occurrence.— Helderbehg Formation, Keyser Member. Cash 



Valley. 



Collection. — U. S. National Museum. 



Uncinulus convexorus n. sp. 

 Plate LXV, Figs. 9-14 



Description.— Shell trigonal or somewhat pentagonal, width at least 

 two-thirds the length. Ventral valve convex, abruptly deflected in front. 

 Beak prominent, pointed, in some forms straight, but in most forms 

 slightly incurved over that of the dorsal valve. Dorsal valve much more 

 convex than the ventral valve, abruptly deflected in front and towards 

 the lateral margins ; beak incurved beneath that of the ventral valve. Sur- 

 face marked by thirty or more low, simple, rounded plications, which 

 become indistinct and are almost obsolete near the beak and are faint near 

 the lateral margins. The lateral margins are slightly inflected so that the 

 sides become distinctly concave. Fold and sinus very faint or absent 

 entirely. The anterior portion is rounded. The plications and depressions 

 interlock to produce sharply serrated lateral and front margins of contact. 

 Dimensions of an average individual : 18 mm. long, and 15.5 mm. wide. 



This form most closely resembles Uncinulus campbellanus of the 

 Helderberg. In the latter species the dorsal valve is elevated near the 

 front into a broad undefined mesial fold, the ventral valve is compressed 

 and a broad sinus occupies the entire width of the narrow front, the front 

 margin curves upward and extends into a subtriangular prolongation. 



In Uncinulus convexorus the dorsal valve is not elevated near the front 

 and there is no distinct mesial fold; the ventral valve is plano-convex. 

 There is an undefined sinus in some individuals while most individuals 

 have no sinus, and the front margin is rounded and sharply serrated. On 



