DENTALIUM. 31 



D. cancellation SOWB. Jun., Thes. Conch., iii, p. 101, pi. 224, f. 36 

 (1860) ; and in Conch. Icon., xviii, pi. 5, f. 29. 



Cancellated near the apical end by distinctly raised striae crossing 

 the longitudinal ribs (Sowb.). 



GROUP OF D. QUADRAPICALE. 



Small or moderate sized shells with the tube square at and near 

 the apex, having angles on the convex, concave and two lateral 

 sides, becoming subcircular at the aperture. Generally costulate be- 

 tween the angles, sometimes smooth ; the apical orifice occupying a 

 short pipe, or without this and square or round. 



Distribution, Pacific shores of tropical and subtropical America, 

 and of the East Indies south to Torres Strait. 



No species of this very distinct group appear in the "Albatross " 

 dredgings off west America, so it is likely that the species are shal- 

 low water forms, not descending to archibenthal or benthal depths. 



No recent species is yet known with certainty from the Atlantic 

 or Gulf of Mexico ; but in the Miocene of Jamaica and San Do- 

 mingo D. dissimile Guppy and its probable synonym, D. pondero- 

 sum Gabb, species allied to D. dipsyche, but much larger and more 

 solid, occur. Dall has described from the Caloosahatchie Pliocene 

 a D. caloosaense, also similar in general characters, and doubtless 

 the direct descendant of the Miocene species. Another similar form 

 has been reported by Harris from the " Galveston deep well " as D. 

 quadrangulare f Sby., from strata of upper Miocene age. The 

 fragments seen by us are not large enough to be characteristic, but 

 it is probably not the recent species. In the same deposit occurs a 

 form not separable so far as material we have seen shows, from the 

 recent D. tesseragonum (D. tetragonum Sby., Harris, Bull. Amer. 

 Paleont, no. 3, p. 13). 



Nothing we have seen from the Eocene belongs to this immediate 

 group. 



Key to Species. 



I. The four primary ribs bifid or trifid ; shell attenuated and stri- 

 ated toward apex, smooth toward aperture. Length about 30 

 mill., dispar, p. 32. 



II. Primary ribs serrate, intervals ribless; aperture angular, 



quadricostatum, p. 33. 



