194 SYSTEMATIC PALEOXTOLOGY 



of the Eocene fauna, as an isolated specimen of this typical Cretaceous 

 species, it seems more probalole that it mnst have been derived from 

 Cretaceous deposits. An even more remarkable occurrence is the dis- 

 covery of many specimens of Terebratula liarlani under similar condi- 

 tions (see p. 204). 



Occurrence. — ^Aquia Foematiox. Clifton Beach. 



Collection. — Maryland Geological Survey. 



Family PTERIIDAE. 



Genus PTERIA Scapoli. 

 Pteeia limula (Conrad). 

 ' Plate LI, Fig. 1. 



Avicula limula Conrad, 1833, Fossil Shells of the Tertiary, No. 4, p. 39. 



Description. — " Shell convex, with slight concentric undulations; 

 umbo tajDering gradually toAA'ards the apex, which is acute, but not 

 prominent; wings large and very oblique; sinus of the posterior margin 

 not profound; nacre very pearly and iridescent. Height, 1^ inches." 

 Conrad, 1833. 



Height of largest fragment, 18 mm. 



Occurrence.^^A'ES'EMOY FoEMATioisr. Popes Creek, Hills Bridge. 

 Aquia Foematiox. Upper Marlboro. 



Collection. — Maryland Geological Survey. 



Family ARCIDAE. 



Genus GLYCYMERIS Da Costa. 



Glycymeeis idoneus (Conrad). 



Plate LI, Figs. 2, 2a, 3, 3a, 4. 



Pectunculus idoneus Conrad, 1833, Fossil Shells of the Tertiary, No. 4, p. 39. 

 Pectunculus idoneus Clark, 1895, Johns Hopkins Univ. Circ, vol. xv, p. 5. 

 Pectunculus idoneus Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 84, pi. xxix, figs, 

 la-lc, 2. 



Description. — " Shell suborbicular, thick, convex; oblique, with rather 

 obscure radiating striae, and very minute, intervening lines; umbo con- 

 vex; beaks distant, rather prominent and pointed; cardinal teeth large; 



