32 DENTALIUM. 



III. Primary ribs not split or serrate. 



a. Surface smooth ; length 20 mill., tesseragonum, p. 34. 

 a'. Surface costulate, 



b. Decidedly attenuated toward the apex ; aperture 

 somewhat compressed vertically, 

 32 riblets at aperture, which is hardly oblique ;. 

 length 45 mill., about 11 times greatest diam., 



dipsycha, p. 33. 



36 riblets at aperture, which is very oblique; 

 length 20 mill., about 6 or 7 times greatest diam., 



quadrapicale, p. 34. 



b'. Not much tapering, the apex large ; aperture cir- 

 cular. 



Length 14 mill., about 8 times the greatest diam., 



fisheri, p. 36. 



Length 20 mill., about 5 J times the greatest diam., 



quadrangulare, p. 35. 



D. quadricostatum Braz., perhaps belonging to this group, is not 

 clearly enough described to admit of its inclusion in the above 

 analysis without doubt. Whether the keels are serrate (as in the 

 fossil D. radula), or divided as in D. dispar, we do not know. 



D. DISPAR Sowerby. PL 4, figs. 52, 53, 54, 55, 56. 



Shell rather slender, the length about 9 times the greatest diam- 

 eter in adults ; earlier third well curved, the rest of the length but 

 slightly arcuate ; much attenuated toward the fine apex ; rather 

 thin, white or bluish-white, glossy and brilliant. Sculpture : four- 

 angled at apex (square in section), the angles dorsal, ventral and 

 lateral, continuing as keels which rapidly become obsolete (extend- 

 ing two-thirds the length of shell in a specimen 15 mill, long, but 

 only about one-third the length in one 30 mill, in length) ; each of 

 the four primary ribs bifid or trifid at summit. Between these 

 angles, throughout their extent, the surface has very fine longitudi- 

 nal riblets ; and not far from the apex a secondary rib arises in each 

 of the four faces, and continues as far as the primary ribs. The 

 larger moiety of the shell is polished, cylindrical, wholly free from 

 longitudinal sculpture. Growth strise fine and inconspicuous. 



Aperture slightly oblique, sub-circular, the peristome thin ; anal 

 orifice square with thin walls, and without slit or notch. Length. 

 30, antero-posterior diam. of aperture 3 mill., lateral 3*2 mill. 



