50 BULLETIN 111, UNITED STATES NATIONAL MUSEUM. 



wide flat intercostal spaces. These primary riblets may continue for 

 some distance without the intercalation of any secondaries, to be 

 eventually supplemented, however, by a few secondaries, or inter- 

 calation may take place very early, even upon the tip, doubling the 

 original nine riblets in number. The secondary riblets assume about 

 equal prominence with the primaries, but at the beginning of the 

 middle third portion of the shell all the sculptural features become 

 fainter, and finally disappear entirely, leaving the second or anterior 

 half of the shell perfectly smooth. An intercostal microscopic reticu- 

 lation exists, formed by 15 to 20 minute longitudinals crossed by 

 equally minute transverse lines, making a delicate network of exqui- 

 site finish. This feature persists only to where the primary riblets 

 dissolve into the smooth polished cylinder. There is.no very definite 

 apical notch or slit observable, but a trace of such is indicated in a 

 few tips on the convex side. 



Length, 30 mm.; diameter, 2 mm. (type). 



Length, 32 mm.; diameter, 2 mm.; arc, 2 (off Habana). 



Length, 56 mm., diameter, 2.5 mm. ; arc, 1.5 (tip gone) (off Habana"). 

 This giant specimen has the anterior three-fourths portion smooth. 



Specimens average about 25 mm. in length. After the senile stage 

 is reached, growth thereafter becomes a variable feature. An average 

 shows in a length of 25 mm., one-third portion of shell ribbed and the 

 remaining two-thirds smooth. 



The type. Cat. No. 95326, U.S.N.M., was obtained by the Blalie at 

 U. S. B. F. Station 2 off Habana, in 805 fathoms, bottom temperature 

 39.75° F. 



There is some variation in the number of riblets in this species, but 

 there is no variation m the original number. The yellow color is 

 very characteristic, but is not always preserved in dead shells, and 

 probably wholly fades in time from specimens collected alive and in 

 perfect condition. From the museum records it appears to be a 

 species of the Antillean continental slope, though occupying a shal- 

 lower zone on the edge of the Gulf Stream off the Florida Keys. 



The variants are indicated in the following two subspecies 

 described in more detail. 



DENTALIUM (ANTALIS) CERATUM CERATUM Dall. 



Plate 7, figs. 2, 4, 5, 6, 7. 



The typical subspecies is restricted to those forms in which the 

 nine primary riblets are not at once increased by the intercalation 

 of secondary riblets. Such additions are but few; generally less than 

 nine. Li this respect only may DentaUum, ceratwm. ceratum be dis- 

 tinguished from the other subspecies enumerated. The type, Cat. 

 No. 95326, LT.S.N.M., is the same as given for D. ceratum. The 

 geographical range also remains the same. 



