EAST AMERICAN SCAPHOPOD MOLLUSKS. 9 



of the mantle cavity, but the escape of fecal and of sexual products 

 is through the smaller end of the shell. The sexes are distinct. 

 The nervous system consists of symmetrically placed central, pleu- 

 ral, pedal, visceral, and buccal ganglia. 



The subgenera of Dentalium. under which the species of the west- 

 ern Atlantic fall are: Dentalium (s. s.), Antalis, Heteroschisma, Fis- 

 sidentalium, GrajHacme, Laevidentalium, Episiylion, BatJioxipJius, 

 Compressidens, and Fustiaria. These will be more particularly 

 discussed under the separate headings for each group, but for a 

 more immediate and general acquaintance the following synopsis of 

 characters is presented. The distinctions can not be sharply drawn, 

 and some species listed under one heading may partake more or 

 less of the characters of some other group. 



SYNOPSIS OF THE SUBGENERA OF DENTALIUM. 



Tip: Angled, polygonal in section. 



Sculpture: Primary ribs prominent; 6, 9, 10, or 12 primary ribs in American, 

 forms. 

 Apical features: A notch or shallow slit on tlie convex side, or apparently 

 wanting 



Size: Medium to large Dentalium, page 22. 



Tip: Less distinctly polygonal, "sometimes round." 



Sculpture: Riblets 6 to 18, sometimes almost obsolete. 



Apical features: Notch (not slit) on convex side or absent. 



Size: Medium or very long and slender Antalis, page 34. 



Tip: Polygonal, American forms, with 9-angled section. 

 Sculpture: Coarse riblets, but not strongly ribbed. 

 Apical features: Slit on concave side. 



Size: Medium to large Heteroschisma, page 56. 



Tip: Indistinctly polygonal, hexagonal. 

 Sculpture: Very many riblets. 



Apical features: Long, narrow slit on convex side. 



Size: Very large Fissidentalium, page 58. 



Tip: Round. 



Sculptiu^e: None. 



Apical features: Apical slit generally on convex side or sometimes laterally 

 placed. 



Size: Small Graptacme, page 65. 



Tip: Round. 



Sculpture: None. 



Apical features: Apical slit generally on convex side. 



Size: Small or long and slender Laevidentalium, page 73. 



Tip: Round. 



Sculpture: None. 



Apical features: No slit, usually a projecting "pipe." 



Size: Very small, needle-like Episiphon, page 77. 



Tip: Unknown. 



Sculpture: None. 



Apical features: Broad slit on convex side. 



Special features: Strongly compressed laterally. 



Bathoxipkus, page 81 . 



