1897.] NATURAL SCIENCES OF PHILADELPHIA. 469 



quite thick and solid. Sculpture : many longitudinal threads about 

 as wide as the iutervals, alternately larger and smaller, crossed by 

 slightly less strong, regular, blunt, encircling stria?, rising into low 

 granules as they cross the longitudinals ; these striae are markedly 

 oblique, bending well forward on the concave and backward on the 

 convex side of the shell, and toward the larger end of adults be- 

 coming irregular and, in part, obsolete. Aperture and apex not 

 preserved, but both orifices are apparently circular. Estimated 

 length 90 mm. in a specimen having a greatest diam, of 8*5 mm. 



A fragment measures : length 36, diam. at larger end 7, at smaller 

 end 4" 7 mm. 



The strongly developed and decidedly oblique encircling sculp- 

 ture is conspicuous and characteristic. In D. carduus and D. callio- 

 glyptum the circular sculpture consists of sharp, raised lamella?; in 

 D. Tryoni of blunt cords, more widely spaced, and with the longi- 

 tudinal riblets, enclosing rhombic depressions (PI. XI, fig. 22). In 

 the imperfect specimen 36 mm. long, measured above, there are 33 

 longitudinal cords and threads at the smaller end, double that num- 

 ber at the larger, where some of the threads are very small. Be- 

 sides the alternation in size, there is a more or less marked teudency 

 for every fourth riblet to be larger, on the median portion of the 

 tube. The largest of the fragments (diam. 8*5 mm.) has about 84 

 subequal longitudinal threads. The increase in number of riblets 

 is by the regular intercalation of a thread in each interval, so that 

 at various ages a specimen would have 16, 32 and 64 riblets ; the 

 increase thereafter being confined to the convex side, where the in- 

 terposed threads appear earliest at each successive increase. 



In the general contour D. Tryoni is not unlike the living D. cap- 

 illosum Jeffr. 



Dentalium dissimile Guppy. PI. XI, figs. 3, 4, 5. 



Dentalium dissimile Guppy, Quart. Journ. Geol. Soc, XXII, p. 292, pi. 17, 

 f. 4 (1866). 



Dentalium ponderosurn Gabb, see below. 



This species, described by Mr. Guppy, from the island of Jamaica, 

 is apparently identical, as Guppy has stated, with a form collected 

 by Gabb in San Domingo. It is a member of the "group of D. 

 quadrapicale " as defined by us in the " Manual of Conchology, 5 — a 

 group distinguished by the quadrangular shape of the apex, the 

 tube having lateral, ventral and dorsal angles posteriorly. Abund- 



5 Vol. XVII, p. 31. 

 31 



