Helderbi<:i?g Fok.aiation in Maryland 23 



veloped mesial fold; beak strongly incurved under tliat of the ventral 

 valve. Surface marked by simple, well-defined, round, radiating plications, 

 which are more elevated and prominent towards the anterior and though 

 not becoming obsolete they are not so prominent in the umbonal region. 

 About 18 to 20 plications on each valve, five of which may occupy the 

 sinus on the ventral valve. The dorsal margins are abruptly deflected 

 posteriorly and become rounded towards the front. 



This form resembles most closely Camarotcechia stricMandi (Sowerby) 

 of the Niagara. It is of about the same size, but differs in having the 

 plications of equal size upon the sides and in the mesial sinus. Sinus nar- 

 rower, no prominent mesial fold. The reference of this species to the 

 genus Camaroto?cliia is not assured as the interior of the shell has not been 

 observed. 



Length about 2.5 cm. ; width about 3 cm. 



Occurrence. — Helderberg Formation, Keyser Member. Devil's 

 Backbone, Cash Valley. 



Collections. — Maryland (jeological Survey, U. S. National Museum. 



Genus UNCINULUS Bayle 



Uncinulus nucleolatus var. axgulatus n. var. 



Plate LXIY, Figs. 18-20 



Description. — Shell subpentagonal in outline, width more than two- 

 thirds the length. Ventral valve convex, not at all gibbous, approaching 

 more to plano-convex; beak sharp, pointed, but slightly incurved and ex- 

 tending beyond that of the dorsal valve. Dorsal valve convex, larger than 

 the ventral valve; beak slightly incurved beneath that of the ventral 

 valve, lateral margins abruptly deflected. Both valves have about equal 

 convexity and are slightly deflected toward the lateral margins, the dorsal 

 valve being more convex is more abruptly deflected toward the lateral 

 margins ; both valves gently curved in front. Surface marked by 23 well- 

 developed simple plications in addition to which there are several indistinct 

 plications near the lateral margins. The plications are very prominent 

 from the center towards the front, becoming obsolete towards the beak. 



