316 SYSTEMATIC PALEONTOLOGY 



Description. — " Shell oblique, suborbiciilar, thin and fragile, ventri- 

 cose; disks with unequal, concentric undulations, forming prominent an- 

 gulated carinas ; concentric striae numerous, prominent ; beaks prominent ; 

 no distinct lunule; cardinal teeth lamellar/' Wagner, 1839. 



Posterior hinge area marked by an angular ridge, posteriorly cuneiform 

 and overlapping a deep, narrow groove and a shallow furrow running 

 backward from the beak ; concentric undulations prominent in interior of 

 young thin shells ; but obsolescent or obsolete in older thickened shells ; 

 pallial sinus large, profound and acutely terminated. 



Length, 61 mm. ; height, 64 mm. ; diameter, 19 mm. 



Occurrence. — St. Mary's Formation. Cdyb Point, St. Mary's River 

 (fide Wagner). Choptank Formation. Governor Run, Sand Hill. 

 Calvert Formation. Hollin Cliff, Wye Mills, Plum Point (Dall). 



Collections. — Maryland Geological Survey, Cornell University. 



Superfamily ISOCARDIACEA. 

 Family ISOCARDIID/E. 



Genus ISOCARDIA Lamarck. 



Isocardia MARKoiii Conrad. 

 Plate LXXXIV, Figs. 2, 3. 



Isocardia Markoei Conrad, lS-t2, Proc. Nat. Inst., Bull, ii, p. 193, pi. ii, fig. 1 (right 



hand figures only and diagnosis in part). 

 Isocardia Markoei Conrad, 1845, Fossils of the Medial Tertiary, p. 70, pi. xi, fig. a 



(right hand figures only and diagnosis in part). 

 Bucardia Markoei Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 576. 

 Glossus Markoei Meek, 186-t, Miocene Check List, Smith. Misc. Coll. (18S), p. 8. 

 Isocardia Markoei Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, p. 1067. 



Description. — " Suborbicular ; length and height nearly equal ; inflated ; 

 umbo very prominent, and the beaks profoundly incurved ; posterior mar- 

 gin direct, arched above, nearly straight below, and obtusely angulated at 

 its junction with the base; base regularly, not profoundly arched; pos- 

 terior slope slightly sinuous." Conrad, 1842. 



Conrad has figured in each case cited above two forms that on compari- 

 son of a number of specimens show constant differences, and his descrip- 

 tion applies partly to one and partly to the other. It becomes necessary, 

 therefore, to restrict his name, and as the remarkable elevation and pro- 



