26 dentalium. 



Group of D. agassizi. 



Forms resembling the D. octangulatum group in general appear- 

 ance, but with more numerous riblets. 



Key to Species. 



a. 11-20 sharp riblets separated by deeply concave intervals, 

 increasing to 24-48 riblets at aperture ; slender, 



agassizi, majorinum. 



a'. About 14 slightly elevated, but well defined riblets ; very 



I? narrow, acuminate ; length 31, diam. 1'6 mill., cookei, p. 29. 



a". 20-25 very delicate, equal lirse ; small, moderately slender ; 



length 31, diam. 2-7 mill. usitatum, p. 29. 



a"\ 17-21 narrow, separated riblets, increasing to about 25 at 



aperture ; conspicuously transversely striated ; large and 



stout; length 75-78, diam. 10-11 mill., shoplandi, p. 28. 



D. AGASSIZI Pilsbry & Sharp, n. sp. PI. 12, figs. 90, 91, 92, 93, 94. 

 Shell gently curved posteriorly, tlie later half nearly straight, 

 tapering, solid, white and lusterless (often with a black encrustation 

 toward the apex, and reddish-brown on the larger end). Sculpture: 

 at the apex there are 12-20 rather sharp and well raised riblets 

 separated by wider, concave intervals ; at a varying distance from 

 the apex an interstitial thread appears in these intervals, so that 

 near the middle of the shell's length there are double that number 

 of riblets and threads, alternately larger and smaller, and at the 

 aperture there are 25-48 unequal riblets and threads, lower and 

 blunter in large examples. Aperture somewhat oblique, subcircu- 

 lar, but the arc along the concave side is sometimes less curved than 

 the remainder of the peristome, and the edge is irregular from 

 breakage. Anal orifice small, circular, no slit or notch, but often 

 the inner layer projects tube-like from erosion of the softer, more 

 chalky outer layer. 



a. Length 65, diam. at aperture, 4'3, at apex 0*7 mill. (type). 



b. Length 31*7, diam. at aperture 3*2, at apex 1 mill. 



c. Length 30*7, diam. at aperture 3, at apex 0*7 mill. 



d. Length 29, diam. at aperture 3"1, at apex 0*7 mill. 



Gulf of Panama, 322 to 1020 fms. ; of Acapulco, 660 fms. ; 

 Santa Barbara Is., California, 414 fms. ; off San Diego, California, 

 822 fms. (U. S. Fiah Commission). 



