DENTALIUM. 19 



the same shell ; usually being 1-3, exceptionally 4, or even 6. My 

 largest example has a length of 50, breadth 4 mill. With the 16 

 six-ribbed specimens, there are 4 with seven primary ribs, the largest 

 one 41 mill. long. These were received with the others, and are 

 exactly like them except in the number of ribs (pi. 2, figs. 23, 24). 

 All the seven ribs run from apex to aperture, and the latter is as 

 pronounced a heptagon as that of the typical form is a hexagon. 



Var. sExcosTATUM Sowerby. PI. 2, figs. 27, 28. 



Shell slender, its smaller half well curved, larger half nearly 

 straight; white. Sculpture of 6 very strongly projecting roimded 

 ribs, about half as wide as their interstices ; the latter on the smaller 

 third of the shell concave and plain, beyond that, one or two inter- 

 stitial threads appear in each interval on the convex side, and later 

 in those on the concave side; these increasing in number until near 

 the aperture there are 3-6 unequal threads on the flat ground of 

 each interval. Aperture hexagonal, the angles more or less project- 

 ing. Anal orifice a minute ovate foramen, excentric in position on 

 the star shaped apex ; no slit or notch. Length 62, breadth and 

 length of aperture 5 mill. 



Japan, Cape Shima, 18 fms. ; Goza Harbor, 6 fms. (St. John) ; 

 China (Sowerby). 



D. sexcostatum Sowb., Thes. Conch., iii, p. 103, pi. 223, f. 11 . 

 (1860) and in Conch. Icon., pi. 2, f 11 (1872).— E. A. Smith, Ann. 

 Mag. N. H., xvi, 1875, p. 113. Conf Lischke, Jap. Meeres- 

 Conch., iii, pp. 74, 75. 



This differs from typical hexagonum in the more sculptured inter- 

 vals, thicker ribs and lai'gersize, but we have little doubt that the 

 forms intergrade. 



D. NEOHEXAGONUM Sharp & Pilsbry, n. sp. PL 11, figs. 74-86. 



Shell decidedly curved toward the apex in the young, only moder- 

 ately arcuate when adult ; slender (the length 12-14 times the great- 

 est diameter, in adults) ; much attenuated toward the apex ; white. 

 Sculpture oi six strong, rounded, projecting ribs, which on the larger 

 half or third of the adult shell become reduced to mere rounded 

 angles ; interstitial riblets wanting, or with one or two low cords 

 developed in each interval toward the larger end of the shell only ; 

 usually with coarse wrinkles of growth on the larger half of the length 

 Aperture hexagonal, but with the angles so rounded as to appear 

 almost circular ; oblique ; anal orifice rounded-oval, without notch 

 or slit. 



