EAST AMERICAN SCAPHOPOD MOLLUSKS. 



57 



This is a subgenus of doubtful value, as it can not be made to 

 include all species possessing the peculiarity of an apical slit on the 

 concave side— its one distinguishing feature. Pilsbry and Sharp ^ 

 have restricted the group to two species, one of which is certainly, 

 and the other doubtfully, of western Atlantic distribution. 



DENTALIUM (HETEROSCHISMA) CALLITHRDC Dall. 



Plate 8, figs. 3, 4, 6. 



1889. Dentalium callithrix Dall, Bull. Mus. Comp. Zool., vol. 18, p. 427, pi. 27, 

 fig. 10. 



1889. Dentalium callithrix Dall, Bull. 37, U. S. Nat. Mus., p. 75, pi. 27, fig. 10. 



1889. Dentalium callithrix Dall, Proc. U. S. Nat. Mus., vol. 12, p. 240. 



1897. Dentalium (Heteroschisma) callithrix, Pilsbry and Sharp, Tryon's Man. 

 Conch., vol. 17, p. 62, pi. 7, fig. 3. 

 The shell is regularly, moderately to strongly curved, with a 

 gradual increase in diameter; it is slightly laterally compressed, 

 causing the oral aperture to be slightly oval. The color is white, 

 porcellanous, but not polished or shining. The tip is nine-angled, 

 with smooth concave intercostal spaces, which later become flatter. 

 At a certain point the intercostal spaces assume, all at the same time, 

 at least three intercalated secondary riblets, which gradually increase 

 in distinctness as the primaries lose, all finally becoming subequal m 

 the anterior portion of the shell. In some cases the primaries retain 

 a slight predominance over the secondaries, even to the aperture. 

 There is no transverse sculpture beyond some hardly observable 

 growth lines, which, toward the aperture, lend a slightly wavy 

 appearance to the riblets. There is no perfectly smooth surface even 

 in senile specimens. A fairly long slit on the concave side is the 



apical feature. t x o oc 



Length, 24 mm.; greater diameter, 2.75 mm.; lesser diameter, 2.25 



mm.; arc, 1.8 type (juv.). ,• + 



Length, 52.75 mm.; greater diameter, 3.25 mm.; lesser diameter, 



3 mm.; arc, 6.5; senile specimen, St. Kitts. 



Length, 28.25 mm.; greater diameter, 3 mm.; lesser diameter, 



2.75 mm.; arc, 2 (juv.), Cape Fear. v . on 



Length, 38 mm.; greater diameter, 3.1 mm.; lesser diameter, 2. J 



mm.; arc, 2 Brazil. i „7 7 + 



The type, Cat. No. 95348, U.S.N.M., was taken by the Blalce at a 

 United States Coast Survey Station not recorded in Yucatan Straits, 



in 640 fathoms. . 



This is a remarkablv constant species of exceptionally wide range. 

 It belongs to the lower zones of the continental^helf and to th^m^ 



1 Tryon, Man. CoDch., vol. 17, p. 61-02. 



