342 SYSTEMATIC PALEONTOLOGY 



Dr. Dall says of it : " The chief characteristics of this species are 

 the long, narrow, somewhat sinuous lunule, tlie straight hinge-line with 

 the shell margin at its ends subangulate, the fine crenulation of the 

 margin of the valves, and the absence dorsally of the rude denticulation 

 due to the surface sculpture from which D. dentata Wood derives its 

 name. 



Occurrence. — Calvert Formation. " Prince George County and 

 elsewhere." (Dall). 



Collection. — U. S. IvTational Museum. 



Superfamily CHAMACEA. 

 Family CHAMID/E. 



Genus CHAMA Liniie. 

 Chama congregata Conrad. 



Plate XCI, Figs. 1, 3, 3. 



Chama congrefjata Conrad, 1833, Anier. Jour. Sci., vol. xxiii, p. 341. 



Chama congregata Conrad, 1838, Fossils of tlie Medial Tertiary, p. 32, pi. xvii, fig. 2. 



Chama congregata Tuomey and Holmes, 1855, Pleiocene Fossils of South Carolina, 



p. 23, pi. Tii, figs. 7-10. 

 Chama congregata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 576. 

 Chama congregata Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), \^. 8. 

 Chama congregata Whittield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 65, pi. ix, figs. 



14-18. 

 Chama congregata Dall, 1903, Trans. Wagner Free Inst. Sci., vol. iii, pt. vi, jx 1400. 



Description. — "Shell sessile, dextral; superior valve a little convex, 

 with numerous, erect, elevated, arched scales; beaks occasionally ros- 

 trated; apex subspiral; scales on the inferior valve broader and more 

 elevated; inner margin crenulated." Conrad, 1833. 



The shape of the lower valve is more or less modified by the surface 

 to which it is attached; within, anterior and posterior muscle impres- 

 sions and pallial line distinct. The curving of the beak to the right as 

 well as the smaller size and much less massive character of the shell, 

 readily separate it from C. corticosa. 



Length, 32 mm.; height, 33 mm.; diameter, 12 mm. 



Occurrence. — Calvert Formation. Church Hill, abundant, and rare 

 at the following localities: 3 miles west of Centerville. 3 miles south of 

 Chesapeake Beach, Plum Point, Truman's Wharf. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



