470 PROCEEDINGS OF THE ACADEMY OF [1897. 



antly developed in the Pacific, this type is not known to have living 

 representatives in the Atlantic or Gulf of Mexico, although Miocene 

 and Pliocene forms have been found in the southern United States. 



In D. dissimile the tube is square at apex (PI. XI, figs. 4, 5), but 

 soon becomes circular in section. Each of the angles at and near 

 the apex is pinched up into a narrow rounded rib. The interven- 

 ing spaces are fiat and plain near the apex, but soon a median 

 thread or pair of threads arises, and a little later other threads ap- 

 pear in the intervals, until there are 30 to 36 threads, varying in 

 size, in the girth of the tube. This sculpture then gradually be- 

 comes weaker, leaving the larger part of the tube cylindrical and 

 smooth, except for circular striation (PL XI, fig. 3). The shell walls 

 are unusually thick. 



Ordinarily a specimen of mature growth loses a great part of the 

 sculptured portion by truncation, so that the square section of the 

 earlier part of the tube is hardly noticeable. This was the case with 

 Guppy's type. The other characters of this species may be seen by 

 reference to the figures here given and to Guppy's original descrip- 

 tion and figure. 



Var. ponderosum Gabb. PL X, figs. 1, 2, 3 ; PI. XI, figs. 15, 16. 



D. ponder osimi Gabb, Trans. Amer. Philos. Soc. (N. Ser.), XV, p. 244 

 (1873). 



Larger, heavier, excessively solid, the cavity reduced to a small 

 perforation by the excessive thickening of the shell. Quadrate form 

 and accompanying sculpture very soon disappearing ; form long 

 and slender. Aperture rather oblique, the peristome, when per- 

 fectly preserved, thin and sharp. 



In this remarkable form the thickness of the shell wall, when an 

 adult is broken across the tube, is greater than the diameter of the 

 orifice. The sculpture disappears sooner than in typical dissimile, 

 and the taper of the shell is very gradual. 



Dentalium Gabbi n. n. PI. X, figs. 6, 7, 13 ; PI. XI, figs. 1, 2. 



D. affine Gabb, Trans. Anier. Philos. Soc. (N. Ser.), XV, p. 244 (1873). 

 Not D. affine Deshayes, 1864, nor of Biondi, 1859. 



Shell slightly curved near the apex, the greater part of the length 

 nearly straight ; thick, solid and strong ; rapidly tapering. Sculp- 

 ture prominent near apex, weak and low toward aperture. At and 

 near the apex, hexagonal with six narrow rounded ribs at the angles, 

 but the symmetry often impaired by the prominence of one or more 

 of the secondary ribs ; the latter lie midway between the six primary 



