288 SYSTEjMATIC PALEONTOLOGY 



Spisula (Hemimactka) curtidens Dall. 

 Plate LXIX, Figs. 12, 13. 



Spisida (Hemimacira) curtldens Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 

 pt. iv, p. 898, pi. xxvii, figs. 2, 24. 



Description.- — " Shell large, not heavy, subtrigonal, with low, narrow, 

 rather pointed beaks, the anterior being markedly longer than the pos- 

 terior end ; surface smooth or striated by incremental lines, and near the 

 base by fine, obscure, irregular longitudinal wrinkles; valves moderated, 

 inflated; anterior end produced, depressed above, rounded in front; pos- 

 terior end short, flattened in front of the beaks, posterior dorsal area im- 

 pressed and bounded by a rounded ridge which extends from the beak 

 to the margin; anterior dorsal area impressed, with a somewhat flexuous 

 surface; hinge with a large but not projecting chondrophore; in the 

 right valve the dorsal laminte are very short and smooth, the cardinal 

 tooth quite compressed. Lon. (of young shell) 22, alt. 17, diam. about 

 9 mm. ; but judging from the fragments found, the species reaches when 

 adult a height and length of 90 mm. 



" This fine Spisula is sharply distinguished from any other American 

 species by its high and triangular form, short, excavated hinge-plate, and 

 the inequilaterality of the shell." Dall, 1898. 



Occurrence. — Choptank Formatiox. Dover Bridge (Dall). Cal- 

 vert Formation. Burch's (Dall). 



Collection. — U. S. National Museum. 



Spisula (Hemimactra) subponderosa (d'Orbigny). 

 Plate LXX, Figs. 1, 2, 3, 4. 



Mactra ponderosa Conrad, 1830, Jour. Acad. Nat. Sci. Phila., vol. vi, 1st ser., p. 228, 

 pi. X, fig. 5,— not of EichAvald, 1830, Nat. Skizze von Lith., p. 207. 



Mactra ponderosa Conrad, 1838, Fossils of the Medial Tertiary, p. 2.5, pi. xiv, tig. 1. 



3Iactra subponderosa d Orbigny, 18.52, Prod. Pal. Strat., vol. iii, p. 100. 



Mactra ponderosa (Jonrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 572. 



Mactra ponderosa Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 11. 



Mactrodesnia ponderosa Conrad, 1869, Amer. Jour. Conch., vol. iv, p. 247. 



Spisula (Hemimactra) subponderosa Dall, 1898, Trans. Wagner Free Inst. Sci., vol. 

 iii, pt. iv, p. 899, pi. xxvii, figs. 3, 16. 



Description. — " Shell subtriangular, convex, thick, concentrically un- 

 dulated ; anterior margin depressed, with an obtuse plication at the angle ; 

 beaks nearest the posterior margin. 



