316 SYSTEMATIC PALEONTOLOGY 



Description. — " Shell oblique, suborbicular, 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 teiminated. 



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



Occurrence. — St. Mary's Eormatton. C£»ve Point, St. Mary's Kiver 

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

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



Collections. — Maryland Geological Surve}^, Cornell University. 



Superfamily ISOCARDIACEA. 

 Family ISOCARDIID/E. 



Genus ISOCARDIA Lamarck. 



IsocARDiA MARKoiii Courad. 

 Plate LXXXIV, Pigs. 2, 3. 



Isocardia Markoei Conrad, 1843, Proc. Nat. Inst., Bull, ii, p. 193, pi. ii, tig. 1 (right 



hand figures only and diagnosis in part). 

 Isocardia Markofi Conrad, 1845, Fossils of the Medial Tertiary, p. TO, pi. xl, fig. 3 



(right hand figures only and diagnosis in part). 

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

 Glossus Markoei Meek, 1864, Miocene Check List, Smith. Misc. Coll. (ISS), p. 8. 

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



Desci-iption. — " 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- 



