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.) 
ARCA IDONEA. Tab 1 , fig. 5. 
Cordate, inequivalve, ventricose, and slightly sinuous; ribs 
about £5, narrow and crenulated; the crenulations most dis- 
tinct 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 subrhomboid al, 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 centenaria. Say , Journ. A. N. S. v. 4, p. 158, 
t. 10; Jig. £. 
Locality. York Town, Ya. Upper Tertiary. 
ARCA INCILE. Table 2, fig. 1. 
Rhomboidal; ribs about £7, 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 - s - 
Locality. James River, near Smithfield, Ya. Upper Tertiary. 
This species varies considerably in form, and young shells 
are often deeply emarginate at the posterior extremity. 
