DENTALIUM-HETEROSCHISMA. 61 



D. belcheri SOWB. jun., Thes. Conch., iii, p. 101, pi. 224, f. 28,29 

 (1860) ; and in Conch. Icon., xviii, pi. 1, f. la, Ib. (1872). 



It is like D. novemcostatum, but with the ribs much more numer- 

 ous. There is a slight articulated appearance on the costae. (Sowb.). 



D. ACULEATUM Sowerby. PI. 10, fig. 66. 



Shell white, strongly curved, unequally striated ; apex attenuated, 

 acuminate, entire. Not unlike the young of D. tarentinum, but more 

 acuminated, and with uneqyal instead of equal striae. (Sowb.). 



Length 21, diam. 3'5 mill, (from fig.). 



Habitat unknown. 



D. aculeatum SOWB., Thes. Conch., iii, p. 100, pi. 225, f. 63 (1860). 

 D. DACOSTTANUM Chenu. PI. 13, fig. 12. 



Shell small, somewhat arcuate, with very numerous and crowded 

 longitudinal striae. (Chenu'). 



Length 7 '5 mill, (from fig.). 



Habitat unknown. 



D. dacostatianum CHENU, Illustr. Conch., i, p. 3. D. dacostianum 

 CHENU, pi. 6, f. 33. 



Evidently a young shell and a doubtful species. The figures show 

 five strong ribs at apex and 16 fine ones at aperture. 



Subgenus HETEROSCHISMA Simroth, 1895. 



Heteroschisma SIMROTH, in Bronn's Klassen u. Ordnungen des 

 Thier-Reichs, iii, Moll., p. 460 (1895). 



Shell coarsely striate or ribbed longitudinally, tapering, and 

 with a apical slit on the concave side. 



An abnormal position of the slit occurs in some other species, such 

 as D. leonince, D. inversum and D. sericatum, belonging to quite 

 different groups. It is no evidence of common origin, so that the 

 group established by Simroth for all species with the slit on the con- 

 cave side, is to that extent an artificial one. If adopted as a sub- 

 generic or sectional name it may be restricted to species of the sub- 

 terfissum type. 



D. SUBTERFISSUM Jeffreys. PI. 7, figs. 15, 16, 17, 18, 19. 



Shell slender and finely tapering, more curved towards the point, 

 rather thin, nearly semitransparent, and glossy; sculpture, from 12 

 to 16 delicate and sharp regular longitudinal striae, which are con- 



