Marylaxd Geological Survey 367 



front to form a slight liiigiuform extension. Dimensions: Length 12.5 

 mm. ; width 9.5 mm. 



Tliis species closely resembles Uncinuhis nucleolatus (Hall) from 

 which it differs in having more angular plications which are more promi- 

 nent towards the front and become obsolete toward the beak. The valves 

 are also less convex than those of that species. 



This form is in the material collected by Bowe and referred by him to 

 the Helderberg. 



Occurrence. — Helderberg Formation? Keyser Member? Cumber- 

 land. 



Collection. — [Maryland Geological Survey. 



[Maynard.] 



UXCINULUS GORDONI n. sp. 



Plate LXV, Figs. 1-6 



Description. — Shell subpentagonal, length and breadth about equal. 

 Ventral valve convex, sometimes slightly depressed convex; beak small, 

 extending beyond and usually slightly incurved over that of the opposite 

 valve. Dorsal valve convex, larger than the ventral valve, at times gibbous. 

 Beak small, incurved under the beak of the ventral valve, more incurved 

 in the more gibbous individuals. Surface marked by simple subangular 

 plications, varying in number from 19 to 36, which extend to the beak; 

 more prominent in front, becoming less conspicuous towards the beak. 

 In some individuals three or four plications form a slight mesial elevation 

 at the anterior portion of the dorsal valve, while two or three plications 

 occupy a shallow sinus in the ventral valve. The sinus and fold become 

 almost obsolete in some individuals at the anterior margin. The lateral 

 margins are deflected somewhat abruptly posteriorly. Dimensions of a 

 small individual: Length 8 mm.; width 7.5 mm. Large individual: 

 Length 12.5 mm. ; width 12 mm. 



This form approaches most closely Uncinuhis nucleolatus (Hall) and 

 Professor Schuchert has suggested that it may be a young form of that 

 species. It differs from Uncinuhis nucleolatus in being smaller, the beak 

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



