1897.] NATURAL SCIENCES OF PHILADELPHIA. 471 



ribs, for the greater part are decidedly smaller than these, and at- 

 tain or fall short of the apex according to the age and consequent 

 degree of posterior truncation. In ihe secondary intervals there 

 arise a variable number of tertiary threads, generally one, two or 

 three in each space ; and at the aperture there is much variation in 

 the number of riblets and threads, different specimens having 36, 40, 

 52, etc. In perfectly preseived shells there is seen an excessively 

 fine but clear cut longitudinal striation in addition to the coarser 

 sculpture described. Growth-strise fine, inconspicuous and oblique. 

 Aperture slightly oblique, circular, the peristome bevelled to a thin 

 edge. Apex rather wide, the orifice subcircular, with a minute 

 notch on the convex side. 



Length 41 '5, diam. at aperture 7'4, at apex 2*3 mm. The largest 

 specimen measures 8 mm. diam. at aperture. 



A large, solid and markedly conical species, with very little 

 curve, and that mainly quite near the apex. It is somewhat like 

 D. disparile on a very large scale, and, as in that species, the num- 

 ber of ribs at the apex is subject to considerable variation, although 

 the fundamental form is hexagonal, the tube soon becoming circu- 

 lar. D. thalloides Conrad of the Claiborne Eocene lacks the fine, 

 clear-cut longitudinal striation of this species, and, moreover, tapers 

 much less rapidly. 



Gabb's diagnosis, published after his death, is not very full, and he 

 gave no figure. As the name imposed by him is preoccupied, we 

 have considered it best to present a detailed description, in proj)os- 

 ing a new name for the form. 



Dentalium haytense Gabb. PI. XI, figs. 8, 9. 



D. Itaytensis Gabb, Trans. Amer. Philos. Soc. (N. Ser. ), XV, p. 244 

 (1873). 



Known only by fragments, the largest of which is probably one- 

 half the original length. These indicate an almost straight, rather 

 rapidly tapering but slender shell, circular or nearly so in section, 

 with smooth, polished surface ; growth-wrinkles light, rather irregu- 

 lar, running somewhat obliquely around the tube ; and there is an 

 occasional constriction so slight as to be hardly mentionable. No 

 trace of longitudinal sculpture. Shell moderately thick (as shown 

 by the section, fig. 8), but becoming very thin at the aperture. 

 Apex unknown. 



Length of type (broken at both ends) 9*4, diam. at larger end 

 1'28 x l - 35, at smaller end 0*68 mm. 



