48 BULLETIN 111, UNITED STATES NATIONAL MUSEUM. 



and slender; the diameter of the shell increases regularly; it is thin 

 but strong and of an opaque white, the surface being porcellanous 

 and without translucent spots or bands. The six angles of the apex, 

 with concave spaces between, become six primary narrow rod like 

 ribs, with separating smooth flat spaces. The ribs become so dis- 

 posed as to leave two broader intercostal spaces on the concave and 

 three narrower spaces on the convex side, one of the ribs being about 

 median on the concave side. Intercalation of secondary longitudinal 

 ribs begins earlier or nearer the apex in the three convex side inter- 

 costal spaces than it does in the two concave side spaces, and is again 

 followed later by still further intercalation ; whereas in the two con- 

 cave spaces the increase in the intercalated ribbing is much less, so 

 that at the aperture, except in very senile specimens, the two con- 

 cave spaces still preserve their definite outline, despite their acquisi- 

 tion of some intercalated ribs. The intercalated or secondary ribs 

 on the convex side assume equal importance with the primaries, 

 but all diminish toward the anterior end, though not losing their 

 identity even in adult shells. There is no transverse sculpture dis- 

 cernible, but rather widely spaced growth lines may be seen with a 

 lens. No apical notch or slit can be detected in any museum speci- 

 mens. 



Length, 14 mm. (Orbigny). 



Length, 32 mm.; diameter, 3.4 mm.; arc, 2.1 (most of tip present), 

 Rio de Janeiro. 



Length, 33 mm.; diameter, 3.5 mm,; arc, 1.75 (tip gone), Rio de 

 Janeiro. 



Length, 26.5 mm.; diameter, 2.9 mm.; arc, 1 (most of tip present, 

 Tampa, Florida. 



No topotypes (Martinique) are in the museum collection, but lots 

 from Rio San Sebastian, Brazil, and from Tampa, Florida, are quite 

 typical. One senile specimen from Sabanilla becomes smooth in the 

 anterior third of shell, the ribs quickly degenerating. 



Further records from the Philadelphia Academy collection are 

 Porto Barrios and Livingston, Guatemala; Belize Harbor; Roseau 

 and Dominica. 



The widely separated geographical records of Brazil and west 

 Florida are not infrequently repeated in other species of mollusks and 

 have already been commented upon under Dentdlium texasianum and 

 D. pseudohexagonum. Were our collection better supplied with shal- 

 low-water records from the Caribbean shore of South America and 

 Central America and to Texas, a chain of connections would no doubt 

 be established, particularly in view of the fact that ocean currents 

 favor such distribution. The species also extends northward into the 

 Lesser Antilles as far as Martinique, but seems not to be known from 



