AiAitvr.wT) (;i:oT,()(;icAL sunvKY 1G3 



I'mnilv CnKBlII.IDAH. 



Gciuis CORBULA Laiiuirck. 

 COTIIUJLA SUBEXGONATA Dall. 



rialu XXXII, Figs. 1, la, 2, 2a, 21). 



Corlinlu niixiiln (lark, IS'.),"), Johns Hopkins Univ. Circ, vol. xv, \>. .">. 



Corbiila namta Clark, 1890, Bull. 141, U. S. Geol. Survey, p. 74. 



Corbida subenijoiuita Dull, 1S!)S, Trans. Waifncr Free Inst. Sci., vol. iii, pt. iv, p. 841. 



Description. — "This foi'iii is siiuilK'i-, Irss iiilhiicil, tliiiiner, and with 

 iiioi'i' nearly jjaralk'] dorsal and ventral Ixirdci's than 6'. alahamiensis. 

 Tlie sculpture is finer lliaii in C eiii/oinihi, wliidi is a more elongated 

 species." Dall, 1898. 



The species was thus defined by Dr. Dall to include the forms from 

 the ('liici<asa\van of Alabama, which had previously been called C. ala- 

 hamiensis Lea or C. alahamiensis var. It is probably the ancestral form 

 of the Claibornian C. alahamiensis Lea. 



This species is yery common in the Maryland and A'^irginia Eocene. 

 Its slightly elevated umbones and the absence of rostrated posterior sur- 

 face separate it from the other species of this genus. In its occurrence 

 it diflt'ers from our other Eocene species of Corhula in that it is most 

 abundant in the lower beds. 



Length, 11 mm.; height, 6 mm. 



Occurrence. — Kanjemoy Foematiox. Woodstock, Popes Creek. 

 Aquia Foemation. Glymont, Clifton Beach, 6 miles east of Washing- 

 ton, Mattawoman Creek, 2 miles below Potomac Creek, 1 mile southeast 

 of Mason Springs, 1 mile northeast of Piscataway. 



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

 LT. S. National Museum. 



CoEBULA ALDEiCHi Meyer. 

 Plate XXXII, Figs. 3, 3a, 4, 4a, 5, 5a, 6, 6a. 



Corbida aldrichi Meyer, 1885, Amer. Jour. Sei., ser. iii, vol. xxx, p. 67. 

 Corbida aldrichi Aldrich, 1886, Bull, i, Ala. Geol. Survey, p. 8.3, pi. 1, fig. 31. 

 Corbula aldrichi Clark, 189.5, Johns Hopkins Univ. Circ, vol. xv, p. 5. 

 Corbida aldrichi Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 74. 



Description. — "It has radiating striae on the umbonial part of the 

 surface, a characteristic which I had not seen before in a Corbula." 

 Meyer, 1885. 



