254 ARGIDJB. 



surface with fine radiating strise. Barhatia lineata, Conrad 

 (cxxvii, fi5). Cret. ; North Carolina. 



AGAR, Gray. Yalves cancellate or costellate ; posterior slope 

 subcarinated or angulated. A. Donaciformis, Reeve (exxvi, 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 sui'face 

 is radiately striated ; the ligamental ai-ea and hinge-teeth are 

 quite similar to those of Barbatia. A. centenaria, Say (cxxvii, 

 66,67). Miocene; U.S. 



PLAGTARCA, Conrad, 1875. Shell transversely trapezoidal, with 

 hinge nearl^'^ 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. Area Garolinensis, Conr. (cxxvii, 79). Cret. 



GRANOARCA, Conrad, 1862. (Cucullsearca, 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. ^.Z^Y/lotZomMS,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). 



SCAPHARGA, Gray, 1847. Shell oval or oblong or svibquad- 

 rangular, thin, radiately ribbed, with a light epidermis; inequi- 

 valve, one valve overlapping the other considerably on the infei'ior 

 margin; teeth subequal, dilated, more or less oblique. A. 

 inequivalvis, Brug. (cxxvi, 44). 



NEMOARCA, Courad, 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. Shell thick, solid, triangular, subcordiform, 

 subequilateral, equivalve; surface with a few large, flat, radiating 



