Maryland Geological Survey 515 



" The dimensions of a large specimen are : Length 22.5 mm. ; height 

 14 mm. ; convexity of one valve 6 mm." Weller, 1903. 



Occurrence. — Helderberg Formatiox, Keysek Mejiber. Tonoloway. 

 Collection. — U. S. National Museum. 



Family LEPERDITELLIDAE 



Genus APARCHITES Jones 



Aparchites gordoni n. sp. 

 Plate XCV, Figs. 1-3 



Description. — Valve rather large, elongate elliptical with an unusually 

 long, straight hinge. Surface convex, with the greatest point of elevation 

 in the middle of the posterior third ; two obscurely defined depressions in 

 the mid-dorsal area. Under a high magnification the surface is seen to 

 be marked by minute, regularly arranged punctae. 



Length 1.60 mm.; height 1.0 mm. 



This new species is based upon a single, well-preserved left valve, pre- 

 senting, as far as it goes, all of the characters of the genus Aparchites. 

 The valve is larger than the average for that genus and thus suggests 

 Leperditella. The long hinge-line, elongate valve, and general convexity 

 and marking of the surface seem to be the specific characteristics — at any 

 rate these features will serve until the characters of both valves are known. 



The specific name is in honor of the late Eobert Gordon, of Cumber- 

 land, Maryland, in appreciation of his work upon the geology of western 

 Maryland. 



Occurrence. — Helderberg Formation, Keyser Member. Cumber- 

 land. 



Collection. — ^U. S. National Museum. 



Family BEYRICHIIDAE 



Genus PRIMITIA Jones and Holl 



Primitia postturgida n. sp. 



Plate XCV, Fig. 4 



Description. — Carapace small, oblong, the ratio of length to height being 



not quite two to one. Valves obliquely subtruncate behind, more rounded 



