380 SYSTEMATIC PALEONTOLOGY 



Pecten septemnarius Conrad, 1840, Fossils of the Medial Tertiary, p. 47, pi. xxii, 



tig. 2. 

 Pecten septenarius Tuomey and Holmes, 1856, Pleioceue Fossils of South Cardhna, 



p. 31, pi. xiii, tigs. 1-4. 

 Pecten septenarius Conrad, 1868, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

 Pecten septenarius Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 4. 

 Pecten Jeffersonius var. septenarius Dall, 1898, Trans. Wagner Free Inst. Sci., vol. 



iii, pt. iv, p. 722. 



Description. — " Shell convex, suborbieiilar : auricles subequal : surface 

 with numerous slightly scaly striae, and about seven remote ribs, of which 

 the three intermediate ones are much elevated, rounded or slightly flat- 

 tened on the top. 



" The strige are equally distinct on the ribs, and in the intermediate 

 spaces. The scales are rather thick, very small, and not confined to the 

 striae, but are also observable in the spaces between the striae." Say, 

 1824. 



In the young the ribs are flat-topped, transversely angular, and as 

 broad across the top as at the base or even broader. In the old the ribs 

 become more rounded transversely. Number of ribs seven or eight. 



Height, 90 mm. ; Avidth, 93 mm. ; diameter, 21 mm. 



Occurrence. — St. Mary's Formation ( ?). St. Mary's River (?). 



Collections. — Maryland Geological Survey, U. S. National Museum. 



Superfamily OSTRACEA. 

 Family OSTREID/E. 



Genus OSTREA Lamarck. 

 OSTREA SELL^FORMIS VAR. THOMASII (Conrad). 



Plate C, Figs. 5a, 5b. 



Ostrca thoinasii Conrad, 1867, Proc. Acad. Nat. Sci. Phila., vol. xix, p. 139 (listed 

 only). 



Descriftion. — Shell small, thin to moderately thick, fan-shaped to 

 pear-shaped in outline; beaks laterally curved; ligament groove exca- 

 vated; ribs on lower valve fifteen to twenty, of thin imbricated scales 

 somewhat elevated; each margin in the lower valve just backward from 

 the hinge line marked by a short punctate impressed line; upper valve 

 thin, slightly convex, surface concentrically marked by the edges of tlie 



