MARYLAND GEOLOGICAL SURVEY 711 



absent; left valve with a deep cardinal socket and a rudimentary posterior 

 tooth ; surface sculpture variable, often discrepant on the two valves of the 

 same individual, usually concentric, never strongly radial ; adductor scars 

 distinct; pallial line indistinct; sinus feeble or obsolete. 



A prominent genus among the small bivalves since the beginning of 

 the Mesozoic. The recent C orb nice include some seventy species of almost 

 universal distribution but more prolific in the warmer waters, particularly 

 in the China seas. 



The Corbulce of the Upper Cretaceous of the East Coast and Gulf are 

 sadly in need of revision. Many of the species have been described from 

 casts and have a doubtful right to stand. The Tertiary and recent Cor- 

 bulce are so difficult to determine with any degree of assurance, even with 

 all their characters preserved, that it seems farcical to attempt to make 

 accurate specific separations from casts of the interior, excepting in 

 unusually well characterized species, such as C. bisulcata Conrad. 



A. Area within the pallial line conspicuously inflated Corbula bisulcata 



B. Area within the pallial line not conspicuously inflated. 



1. Valves very strikingly dissimilar, right valve highly inflated, very 



coarsely plicated concentrically. .Corbula crassiplica 



2. Valves not very strikingly dissimilar. 



a. Radial sculpture absent. 



i. Latitude of adult shell exceeding 9 mm., concentric 

 sculpture fine, sharp, crowded. 



Corbula monmouthensis 



ii. Latitude of adult shell not exceeding 9 mm., concen- 

 tric sculpture more or less obtuse, 

 a'. Valves not conspicuously compressed. Concentric 

 plications exceeding 25 in number. 



Corbula terramaria 



b'. Valves conspicuously compressed. Concentric pli- 

 cations not exceeding 25 in number. 



Corbula percompressa 



b. Radial sculpture present upon the disk in the form of faint 



linear striations Corbula subradiata 



CORBULA BISULCATA Conrad 



Corbula bisulcata Conrad, 1875, Kerr's Geol. Rept. of North Carolina, App., 



p. 11, pi. ii, figs. 13, 14. 

 Corbula foulkei Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 180, pi. 



xxiii, figs. 27-29. (Not C. foulkei Lea.) 

 Corbula bisulcata Weller, 1907, Geol. Survey of New Jersey, Pal., vol. iv, 



p. 638, pi. Ixxii, figs. 15-22. 



