DENTALIUM-RHABDUS. 115 



exactly the size and figure of D. dalli, but it is a very solid shell, 

 while our new form is one of exceptional fragility, and moreover 

 lacks the apical striation of young pretiosum. 



The range of Ball's tusk-shell extends in deep water the entire 

 length of the North American continent, though apparently more 

 numerous in the north. We have distinguished it by a specific name 

 which so long as West Coast shells are studied, will be an honored 

 one among naturalists. 



Imperfect specimens, apparently of this species, were obtained by 

 the Fish Commission in the Gulf of Panama. Types are no. 107696 

 U. S. Nat. Mus. 



Group of D. perception. 

 D. PERCEPTUM Mabille & Rochebrune. 



Shell rather straight, scarcely incurved, densely concentrically 

 striated, slowly increasing. White, under a caducious bright chest- 

 nut epidermis ; diaphanous, fragile, scarcely shining, apex entire, 

 nearly tubular. Length 45-50, greatest diam. 8 mill. (M. & R.}. 



Cape Horn. 



Dentalium pereeptum MAB. & ROCH., Miss. Sci. Cap Horn, vi, 

 ZooL, Moll., p. 99 (1889). 



An elongated, straight shell, presenting somewhat of a very notice- 

 able curvature, delicately ornamented with fine concentric striae, and 

 pure white under a fugitive, very light chestnut epidermis ; diaphan- 

 ous, fragile, without luster, and of slow increase ; the summit entire, 

 a little in the shape of the mouth of a huntsman's horn. (M. & jR.). 



Group of D. eburneum. 



Shell thin, moderately or slightly curved, glossy, with numerous 

 unequal coarse annular wrinkles ; anal orifice simple. 



D. EBURNEUM Linne. PI. 20, figs. 38, 34. 



Shell long and slender, moderately arcuate, attenuated posteriorly, 

 the length about 12? times the diam.; rather thin ; white, shining. 

 Sculpture of numerous, unequal, irregularly spaced encircling ribs, 

 rather low and rounded, with fine growth-lines throughout; some- 

 times showing traces of longitudinal striation in places. Aperture 

 subcircular or rounded-ovate, narrower toward the concave side ; 

 peristome thin. Anal orifice small, rounded-oval, the greatest diam- 

 eter antero-posterior ; a slight notch on the convex side or none. 



