MARYLAND GEOLOGICAL SURVEY 389 



"Ribs somewhat flattened, as wide or rather wider than the interven- 

 ing spaces ; the whole surface concentrically wrinkled : umhones not re- 

 markably prominent: apices remote, the intervening space rhomboidal, 

 with continued indented lines, arcuated under the apices : hinge margin 

 perfectly rectilinear, angulated at the extremities, the anterior one a 

 little projecting: teetJt with a continued, uninterrupted line, parallel, 

 excepting at the two extremities of the line, which decline a little, and 

 the teeth are there decidedly longer and oblique with respect to the 

 others of the range: posterior end obliquely rounded to the base: liase 

 nearly rectilinear and parallel to the hinge margin, and deeply crenated 

 on the inner margin: anterior end produced below the middle, and 

 rounded, and a little contracted near the superior angle." Say, 1824. 



Length, 55 mm. ; height, 34 mm. ; diameter, 16 mm. 



Occurrence. — St. Mary's Formation. St. Mary's Eiver (quite rare). 



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



Arca (Scapharca) idonea Conrad. 

 Plate CVI, Figs. 1, 2. 



Area idonea Conrad, 1833, Fossil Shells of the Tertiary, p. 16, pi. i,'fig. .5. 



Arca stillicidum Conrad, 1832, Fossil Shells of the Tertiary, p. 1.5, pi. i, flg. 3 



(young). 

 Area idonea Conrad, 1840, Fossils of the Medial Tertiary, p. 5.5, pi. xxix lig. 3. 

 Arca stillicidum Conrad, 1810, Fossils of the Medial Tertiary, p. .5.5. 

 Area idonea Emmons, 1858, Kept. N. Car. Geol. Survey-, p. 285. 

 Latiarca idonea Conrad, 1862, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 289. 

 Scapharca idonea Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 579. 

 Scapharca idonea Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 6. 

 Area (Latiarca ?) idonea ? Whitfield, 1894, Mou. xxiv, U. S. Geol. Survey, p. 47, 



pi. vii, fig. 1. 

 Scapharca (Scapharca) idonea Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 639. 



Description. — "Cordate, inequivalve, ventricose, and slightly sinuous; 

 ribs about 25, narrow and crenulated ; the crenulations most distinct on 

 the larger valve; beaks very prominent and distant; area with undulated 

 grooves; hinge with the series of teeth contracted in the center, and a 

 little decurved at the ends." Conrad, 1832. 



The shell is thick and large with a more or less sharply angular 

 slope from the beak to the posterior extremity of the base. Near this 

 angulation the ribs of both valves are finely striated. The more angular 



