18 DENTALIUM. 



Tersely striated ; apex rather large, entire, without a slit. Length 

 72, diara. 6 mill. (Dkr.~). 



Japan (Dkr.). 



D.japonieum DKR., Malak. BI., xxiv, p. 68 (1877); Index Moll. 

 Mar. Jap., p. 153, pi. 5, f. 2. 



Not unlike D. javanum Sowb., but easily distinguished by being 

 more slender, with stronger interstitial riblets and the whole sculpt- 

 ure more prominent. (Dkr.~). 



It is evidently near D. octangulatum, but the diameter increases 

 less rapidly, the aperture being contained twelve times in the length. 

 Not seen by us. 



D. HEXAGONUM Gould. PI. 2, figs. 20, 21, and var. 23, 24. 



Shell long, slender, bony, arcuate, hexagonal with obtuse, later- 

 ally compressed angles, the interspaces unsculptured ; peristome 

 six-angled. Length 55, diam. 4 mill. (Gld.~). 



Hongkong (N. Pacif. Expl. Exped.) ; Singapore (Sowb., Archer !) ; 

 Bay of Yeddo, Japan (Lischke). 



D. hexagonum GLD., Proc. Bost. Soc. N. H. vii, p. 166 (Dec., 

 1859); Otia Conch., p. 119. SOWB., Thes. Conch., iii, p. 103, pi. 

 223, f. 10 (1860) and in Conch. Icon., xviii, pi. 2, f. 6 (1872). 

 LISCHKE, Jap. Meeres-Conchyl. iii, p. .74, pi. 5, f. 4, 5 and var., f. 6, 

 7 (1874). 



Not " D. hexagonum Sby." of Carpenter and other authors on 

 West American mollusks. 



Several specimens of the original lot are before us, with others 

 collected by Archer at Singapore. These have sharply cut, high and 

 rather narrow ribs, separated by flat or concave intervals showing 

 lines of growth only, although in the largest there are faint traces 

 of interstitial striae. One out of 5 from China is 7-ribbed (see be- 

 low). There is no apical slit. The shell is somewhat more slender 

 than D. octangulatum, the diam. of aperture being contained 12 to 

 14 times in the length of shell. 



Lischke writes as follows : Gould's diagnosis agrees excellently 

 with the 16 examples before me, except that the intervals are plain 

 only in the smaller part of the shell to about 20-25 mill, from the 

 .apex, beyond there having very fine interstitial riblets. These riblets 

 become more numerous toward the aperture, but they vary in num- 

 ber, not only in different specimens but in the different intervals on 



