22' BULLETIN 111, UNITED STATES NATIONAL MUSEUM. 



SYSTEMATIC CONSIDERATION OF THE SPECIES.^ 



Genus DENTALIUM Linnaeus. 



Subgenus DENTALIUM Linnaeus, 1758. 

 1758. Dentalhim Linnaeus, Syst. Nat., ed. 10, p. 785. 



Type. — D. elepJiantinum Linnaeus. 



Tip polygonal ; in the American species observed clearly hexagonal 

 or if obscurely angled then giving rise to 9, 10, or 12 primary longi- 

 tudinal ribs usually increased by intercalation of secondary ribs to 

 a total rarely exceeding 20 altogether. The ribs are usually well 

 developed and prominent, especially upon the posterior portion of 

 the shell. Microscopic sculpture including both longitudinal and 

 transverse series may or may not exist. Apical features are gener- 

 ally wanting but in some species a slight notch is observable on the 

 convex side. 



The shells of this subgenus- are medium to fairly large in size — 

 from 21 to 89 mm. in American forms. 



The line of differentiation between this and the next subgenus — 

 Antalis is ill defined. In general the latter group includes smaller 

 and less obviously sculptured shells. The American species of 

 Dentalium may be roughly grouped into those of 6 primary ribs 

 and those of more than 6, and also those with and those without 

 microscopic sculpture. 



KEY TO THE SPECIES OF THE SUBGENUS DENTALIUM. 



Tip section hexagonal. 



Intercostal spaces smooth. 



Ribs rod-like and slender texasianum, page 27. 



Intercalated ribs few. 



Intercostal spaces flat texasianum texasianum, page 28. 



Intercostal spaces rounded texasianum rioense, page 29. 



Intercalated ribs numerous texasianum cesium, page 29. 



Ribs not rod-like, broader rebeccaense, page 31. 



Intercostal spaces not smooth. 



Intercostal spaces microscopically engraved gouldii, page 29. 



Shell strongly curved. 



Intercalated secondary ribs present gouldii colonense, page 31. 



Intercalated secondary ribs absent. . .gouldii portoricense, page 30. 

 Shell not strongly curved gouldii gouldii, page 30. 



1 111 the description of species following the word "posterior" refers to that portion of the shell near the 

 smaller or pointed end ; "anterior" to that portion of the shell near the aperture or larger end. The"pri- 

 mary " ribs or riblets are the first or original longitudinals appearing on the tip of the shell; the " secondary " 

 ribs or riblets are those later appearing or intercalated between the "primaries." The "arc" of the shell 

 is a measure of curvature determined by the distance in millimeters from a line connecting the apex and 

 aperture to the highest point above it in the concave arch of the shell. The drawing shows this more 

 clearly than words. 



