78 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



2. polynema, Conrad. 



Shell transversely trapezoid-subovate, narrowing anteriorly; car- 

 dinal margin descending forward, and meeting the anterior and poste- 

 rior margins at more or less defined angles; cardinal area extremely 

 narrow, with about two deep, divaricating cartilage-furrows; lateral 

 denticles very oblique, those on the posterior side more elongated and 

 striated; surface with fine radiating stria 1 . — Barbatia lineata, Conrad. 



3. acar, Gray. 



Shell generally small, gibbous, subovate; hinge short; area usually 

 very small, with cartilage-furrows few, longitudinally, or some- 

 what divaricately arranged; surface cancellately striate or costate ; 

 posterior dorsal slopes subcarinate. — B. divarkata, Sowerby. 



4. calloarca, Cray. 



Shell of much the form of the last, but with posterior dorsal 

 slopes strongly carinate, and anterior and posterior portions occu- 

 pied by strong radiating costas that impart a crenate character to 

 the inner margins behind; area very narrow, with cartilage furrows 

 as in the last. — B. altervata, Reeve. 



5. striarca, ( Jonrad. 



Shell transversely oblong-oval, with rounded extremities and a 

 deeply sinuous basal margin; surface radiately striate; area moderate; 

 cartilage-furrows very small, crowded, and crossing the area at right 

 angles to the cardinal margin; hinge rather distinctly arched, with 

 denticles hollow. — Area centeharia, Conrad. 



G. plagiarca, Conrad. 



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 compara- 

 tively large and slightly angulated in the middle; surface radiately 

 costate; area very narrow, and marked by numerous minute, crowded, 

 divaricating cartilage-furrows. — Area Carolinensis, Conrad. 



I 7. granoarca, Conrad (zz ? Cucullcearca, Conrad). 



Shell sometimes attaining a large size, transversely oval-subtrape- 

 zoidal, with cardinal margin more or less rounding into the laterals; 

 area rather broad and very oblique, with divaricating cartilage-furrows; 

 cardinal margin very wide posteriorly; denticles smaller and more 



