16 ;..".:. 



valve plain, except those on the anterior side, which are cren- 

 ulated ; beaks prominent ; margin profoundly crenate. 



Locality. St. Mary's River, Md. Upper Tertiary. 



It resembles A. incongrua, Say (A. nodosa, Wood.) 



ARC A IDONEA. Tab 1, fig. 5. 



Cordate, inequivalve, ventricose, and slightly sinuous ; ribs 

 about 25, narrow and crenulated ; the crenulations most dis- 

 tindt on the larger valve; beaks very prominent and distant; 

 area with undulated grooves; hinge with the series of teeth 

 contracted in the centre, and a little decurved at the ends. 



Locality. St. Mary's River. Md. Upper Tertiary. 



ARCA CENTENARIA. Table 1, fig. 4. 



Oval or subrhomboidal, obsoletely contracted at base, with 

 numerous radiating striae, alternating in size ; anterior and 

 posterior margins obtusely rounded ; beaks approximate, not 

 prominent ; area much contracted and transversely sulcated ; 

 series of teeth decurved at the extremities ; teeth obsolete under 

 the beaks ; muscular impressions a litle raised, with a groove, 

 along the sides ; margin entire. 



Syn. ARCA CENTENAKIA. Say, Journ. A. N. S. v. 4, p. 138, 

 t. 10, fig. 2. 



Locality. York Town, Va. Upper Tertiary. 



v 



ARCA INCILE. Table 2, fig. I. 



Rhomboidal ; ribs about 27, finely granulated and alternated 

 with fine lines posterior to the middle of the valve ; beaks near 

 the anterior end ; not prominent ; apex acute ; area with trans- 

 verse lines on the anterior portion, behind which is a single 

 oblique groove extending from the apex to a little behind the 

 middle ; series of teeth rectilinear ; margin crenate. 

 Syn. ARCA INCILE, Say, Journ. A. N. S. v. 4, p. 139, t. 10, 

 fig.*. 



Locality. James River, near Smithfield, Va. Upper Tertiary. 



This species varies considerably in form, and young shells 

 are often deeply emargmate at the posterior extremity. 



[80] 



