820 PROCEEDINGS OF THE NATIONAL MUSEUM. voi,xx. 



left valve) ; the postero-dorsal margin is usually slightly concave, but 

 is sometimes nearly straight, and in some cases decidedly concave, and 

 slopes gradually to the superior aiigulation of the rostrum; the liga- 

 mental area is narrow, deep, and long, extending for nearly the entire 

 length of the dorsal margin, and is clearly denned by a marginal ridge 

 which is sharper on the left valve. In most specimens this valve is 

 marked by a slight, ill-defined groove running from the beak to the 

 antero- ventral margin, where it often forms a slight emargination, but 

 is often scarcely discernible, except by the change in the direction of the 

 lines of growth and character of the epidermis. The surface of the 

 left valve is covered with irregular concentric ridges and rather uneven 

 lines of growth; the right valve usually shows rather regular, concen- 

 tric undulations on which are numerous fine, pretty regular, lines of 

 growth; this valve is also usually marked by faint, and rather indis- 

 tinct, radiating, impressed lines which are more or less broken and 

 often branched or forked; these are scarcely visible without a lens. 

 Epidermis thin, brownish yellow, usually mostly peeled off in dry speci- 

 mens, but on the anterior end, in front of the radial groove, it is a 

 little more persistent. In the left valve the anterior tooth is strong 

 and prominent, with the proximal end the thicker, more elevated, 

 rounded or clavate; distally it is curved and diverges considerbly 

 from the antero-dorsal margin; between this tooth and the resilial pit, 

 there is a small central tooth only a little elevated. The resilial pit 

 is directed obliquely backward, and its cavity is obliquely upturned, 

 forming a distinct excavation on the inner surface of the posterior 

 tooth with which it is confluent; this posterior tooth is simply a dis- 

 tinctly thickened and slightly elevated portion of the postero-dorsal 

 margin, which forms the boundary of the ligamental area, it is often, 

 but not always, opposite the position of the resilial pit. In the right 

 valve the anterior tooth is a slightly raised, somewhat curved ridge^on 

 the inner .surface of the shell, running to the middle of the anterior 

 muscular scar; the central tooth is shorter and much more elevated, 

 most prominent at its inner end; between these two teeth there is an 

 additional, slightly raised, tooth-like ridge; the posterior tooth is about 

 as long as the anterior, and much more elevated and stouter, its distal 

 end being the thicker and higher, with a distinct angular summit; the 

 oblique resilial pit is excavated out of its anterior surface. The ossicle 

 is somewhat elongated, curved or crescent shaped. The interior of the 

 shell is only slightly lustrous and shows but little iridescence. 



Length of one of the largest specimens, 19 mm.; height, 11 mm.; 

 thickness, about 3 mm. 



Found in considerable numbers at twenty-three stations, north of 

 Cape Cod, off Stellwagens Bank, and off Chatham, in 10 to 43 fathoms, 

 18712-1881. 



This species, which is common in the vicinity of Cape Cod, has prob- 

 ably been confounded, hitherto, with C. trilineata Say, and C. goul- 

 diaiia Dall. From the latter, which occurs abundantly in the same 



