20 DENTALIUM. 



Length 30'5, diam. of aperture 2'5 mill. ; height of arch from 

 chord 4'8 mill. 



Length 31'5, diam. of aperture 2'3 mill.; height of arch from 

 chord 3-7 mill. 



Santa Barbara, San Pedro Bay, San Diego, California (south to 

 Acapulco ?). Fossil in southern Californian Pliocene. 



" D. hexagonum Sby.," CPR., Suppl. Rep. Moll. West Coast N. A., 

 in Rep. Brit. Asso. Adv. Sci. for 1863, pp. 612, 648 and 668 (" D. 

 f hexagonum, var. B ") ; and in The Mollusks of Western North 

 America, Smiths. Misc. Coll., no. 252, pp. 98, 134, 154 (1872). 

 GABB, Palseont. of Calif., ii, 1869, p. 86. WILLIAMSON, Proc. IT. 

 S. Nat. Mus., xv, p. 194 (1892). KEEP, West Coast Shells, p. 114. 



This species has hitherto been confused with the Japanese Z>. 

 hexagonum Old., a mistake apparently originating with Dr. Car- 

 penter. It never grows as large as that form, the six primary ribs 

 lose conspicuously in prominence on the larger part of the shell, and 

 fewer interstitial riblets develop. The two species are very readily 

 separated at all stages of growth, and have only been united because 

 no comparison of Oriental and Californian specimens seems hitherto 

 to have been made. 



The young shells, as usual, are much more curved and taper more 

 rapidly than adults. Specimens from the Pliocene at San Diego are 

 larger than any recent shells we have seen. 



There is a form of this species having 7 or even 8 ribs, (pi. 11, 

 figs. 81, 82, 83, 86) and another with a short apical slit on both con- 

 vex and concave sides of the tube (pi. 11, fig. 84). We have not 

 seen specimens enough to be satisfied that these are more than 

 variations of D. neo hexagonum. 



D. GOULDII Dall. PI. 7, fig. 14 (var. obscurum'). 



Shell elongated, slender, slightly arched, vitreous, anteriorly whit- 

 ish, behind with a yellowish or pale greenish tinge , surface polished, 

 with fine microscopic longitudinal strise over a large part of the 

 surface; in well developed specimens the shell is hexagonal and 

 six-sided, with the sides impressed so that the ribs stand out like 

 marginating rods ; as the shell grows older, the angles become less 

 marked, although generally quite perceptible at the aperture ; the 

 lines of growth are visible as extremely fine engraved striae ; in an- 

 other mutation of the species (which served the draughtsman for 

 fig. 14), there are longitudinal threads between those forming the 



