254 ARCIDA. 
surface with fine radiating strie. Barbatia lineata, Conrad 
(exxvii, 65). Cret.; North Carolina. 
ACAR, Gray. Valves cancellate or costellate; posterior slope 
subcarinated or angulated. A. Donaciformis, Reeve (cxxvi, 41). 
CALLOARCA, Gray, 1857. Shell with the posterior slope strongly 
carinated, and that and the anterior slope both strongly ribbed, 
the ribs forming marginal teeth within. A. alternata, Reeve 
(cxxvi, 42). 
STRIARCA, Conrad, 1862. Shell oblong, with a depression 
running from the beak to the ventral margin, which is somewhat 
insinuated, no doubt corresponding to a small gape; the surface 
is radiately striated; the ligamental area and hinge-teeth are 
quite similar to those of Barbatia. A. centenaria, Say (exxvii, 
66,67). Miocene; U.S. 
PLAGIARCA, Conrad, 1875. Shell transversely trapezoidal, with 
hinge nearly straight, and meeting the lateral margins at more 
or less defined angles; lateral denticles very oblique, one or two 
of ‘the anterior series being comparatively large and slightly 
angulated in the middle; surface radiately costate; area very 
narrow, and marked by numerous minute, crowded, divaricating 
cartilage-furrows. Arca Carolinensis, Conr. (cxxvii, 79). Cret. 
GRANOARCA, Conrad, 1862. (Cucullearea, Conrad, 1865.) Equi- 
valve, gaping anteriorly ; hinges rather wide and very oblique, 
with longitudinal grooves angulated under the back; tooth more 
or less divided into granular plates, posteriorly widely expanded 
and broken into irregular granules. A. propatula, Conr. (cxxix, 
5). Miocene; U.S. 
LITHARCA, Gray, 1840. Shell cuneiform, obliquely truncated 
behind, elongated and rounded in front, gaping inferiorly ; no 
teeth behind the beaks. A.lithodomus,Sowb.(cxxvii,68). Recent. 
ANOMALOCARDIA, Klein, 1753. (Anadara, Gray, 1847.) Shell 
thick, subcordiform or subquadrangular, equivalve, subequi- 
lateral; valves radiately costate, with a smooth or rugose olive 
epidermis ; hinge-line nearly straight, with numerous teeth, 
smallest in the middle, largest at the extremities. Differs from 
Barbatia principally in form. A. auriculata, Lam. (cxxvi, 43). 
SCAPHARCA, Gray, 1847. Shell oval or oblong or subquad- 
rangular, thin, radiately ribbed, with a light epidermis; inequi- 
valve, one valve overlapping the other considerably on the inferior 
margin; teeth subequal, dilated, more or less oblique. <A. 
tnequivalvis, Brug. (exxvi, 44). 
NEMOARCA, Conrad, 1869. Shell elongately trapezoidal, sub- 
equilateral, tumid, radiately ribbed; ligamental area narrow; 
hinge-line straight, with small transverse teeth. A. cretacea, 
Conrad (exxvi, 48). 
SENILIA,Gray,1847. Shellthick,solid,triangular,subcordiform, 
subequilateral, equivalve; surface with a few large, flat, radiating 
