MARYLAND GEOLOGICAL SURVEY 677 



Type Locality. Owl Creek, Tippah County, Mississippi. 



Conrad's figure somewhat belies his description. The shell is rather 

 small and thin, subcircular and feebly convex; the umbones are rather 

 gibbous for the genus, the apices acute and prosogyrate, rising above the 

 dorsal margin and approximately central, not terminal in position. The 

 area of maximum inflation extends obliquely backward from the umbones 

 to the posterior ventral margin widening toward the base, thus giving 

 to the shell the characteristically oblique aspect which inspired the name. 

 The anterior end is broadly and symmetrically rounded, the posterior 

 obscurely truncated, the base line arcuate. The external surface is sculp- 

 tured with very fine, overlapping concentric laminae, most sharp and most 

 regular in th eumbonal region and along the anterior margin. The char- 

 acters of the interior are not known. 



The species is represented in Maryland by a single imperfect valve. 



Occurrence. MONMOUTH FORMATION. Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collection. Maryland Geological Survey. 



Outside Distribution. Ripley Formation. Exogyra costata zone, 

 Tippah County, Mississippi. 



Genus CYCLINA Deshayes 

 [Traite Elem., vol. i, 1849, p. 623] 



Type. Venus sinensis Gmelin = Venus chinensis Gmelin. 



" The dental formula is L - 010101Q . The fourth, or posterior, right 



E. 1010101 



cardinal is nearly obsolete; the one in front of it and the anterior left 

 cardinal are bifid. The shell is suborbicular, nearly equilateral, and 

 plump; the ligament uncovered but deep-set; there is neither a defined 

 lunule nor escutcheon, the sculpture is faint and chiefly concentric, feebly 

 reticulated by radial stria ; the hinge plate is broad, the inner margins of 

 the valves crenulate, the pallial sinus moderate in size, acutely angular in 

 front, and obliquely ascending. There is no trace of lateral teeth; the 

 periostracum is polished and translucent, the coloration tinted, without 



Etymology: /a'/cXcs, circle. 



