164 SYSTEMATIC PALEOXTOLOGT 



" Eoundecl trigonal; ventricose; posterior side carinated; beak small, 

 curved anteriorly, in the left valve nearly in the middle; right valve 

 briefly rostrated; in both valves the iimbonial part is without concentric 

 ribs, but with impressed, radiating lines — the ventral part with con- 

 centric ribs." Aldrich, 1886. 



This remarkable form, with its radiating lines upon the umbonial 

 portions of the shell, is very common. The Maryland form differs in 

 no essential particulars from the Gulf type. 



Length, 11 mm.; height, 7 mm. 



Occurrence. — ISFaxjemoy Foematiox. Upper Marlboro (deep cut near 

 Chesapeake Beach E. E. station). East and west of Port Tobacco, Head 

 of ISTanjemoy Creek, ^ mile below Chapel Point, Popes Creek, If miles 

 above Popes Creek, 2^ miles above Popes Creek, Woodstock, 1 mile 

 southeast of Piscataway. Aquia Foematiox. Aquia Creek, Liverpool 

 Point, Clifton Beach. 



Collections. — Maryland Geological Surve}', Johns Hopkins University, 

 U. S. National Museum. 



CoEBULA oxiscus Conrad. 

 Plate XXXII, Figs. 7, 7a, 8, 8a, 8b. 



Corhula oniscus Conrad, 1833, Amer. Jour. Sci., vol. xxiii, p. 341. 

 Corbula oniscus Clark, 1895, Johns Hopkins Univ. Circ, vol. xv, p. .5. 

 Corhula oniscus Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 7.5. 

 Corbula (Aloidis) onisctis Dall, 1898, Trans. Wagner Free Inst. Sci., toI. iii, pt. iv, 

 p. 843. 



Description. — " Shell elevated; larger valve ventricose, with profound 

 sulci terminating at the umbonial slope, which is carinated; posterior 

 extremity narrowed and truncated, from the posterior angle of Avhich 

 a carina extends to the apex, nearly parallel with that of the umbonial 

 slope; superior valve concentrically striated. Length, one-third of an 

 inch." Conrad, 1833. 



This species, characterized by its solid form and numerous concentric, 

 wrinkled, approximate lines and prominently rostrated posterior sur- 

 face, is common at several localities in the Maryland Eocene. Xone of 

 the specimens obtained reach the size of the larger individuals in the 

 Gulf Eocene. 



