524 Systematic Paleontology 



the anterior lateral margin than into the posterior ; area so clearly defined 

 in most of the Ledidce indicated more by a change in the direction of the 

 growth lines than in the plane of the shell; external surface faintly 

 striated with irregular incrementals least faint upon the anterior portion ; 

 hinge taxodont but nothing further known about the characters of the 

 interior. 



Dimensions. — Altitude 10.3 mm.; latitude 28 ± mm.; diameter of 

 double valves 2.9 mm. ; distance from umbones to anterior margin 6.5 mm. 



This species is well characterized by the elliptical outline, the sub- 

 parallel posterior dorsal margin and base line, and the irregularity of the 

 faint incremental striae. Perissonota protexta Conrad is alate rather than 

 elliptical, and the incremental sculpture is sharp and regular. The 

 species is described from double valves from which a small piece of shell 

 substance has been chipped away at the hinge line revealing the taxodont 

 dentition. 



This species is named for Dr. Homer P. Little, formerly with the 

 Maryland Geological Survey and now at Colby College. 



Occurrence. — Monmouth Formation. Brightseat, Prince George's 

 County. 



Collection. — Maryland Geological Survey. 



Superfamily ARCACEA 

 Family PARALLELODONT1DAE 

 Genus NEMODON Conrad 

 [Am. Jour. Conch., vol. v, 1869, p. 97] 

 Type. — " Nemodon eufalensis " Conrad = Nemodon conradi Johnson. 

 Not Area (Macrodon) eufalensis Gabb, 1860. 



" Equivalved, thin ; hinge line long and straight, or slightly curved 

 under the umbo; hinge in left valve with three linear teeth parallel with 

 the anterior cardinal margin ; posterior lateral tooth double, very long, 

 linear; under the apex a few granular teeth." — Conrad, 1869. 



Etymology: Nemodon, from vij/ia, thread, and 68oi>s, tooth. 



