DENTALIUM. 



IV. Nine ribbed, the ribs sometimes obsolete near aperture. 



a. Ribs rounded, distant, interstices flat; length 20 mill., 



8 times the diam., robustum, p. 12. 



a'. Ribs slender, intervals unequal, shallow, with slight 



threads ; length 30 mill., 8 times the diam. (Japan), 



yoholmmense, p. 16. 

 a". Ribs strong, equidistant, surface finely lineolate; 

 length 40-48 mill., about 7 times the diam. (Suez), 



lineolatum, p. 11. 

 a'". Ribs strong, wide, unequally spaced, surface finely 

 striated ; length 45 mill., about 11 times the diam. 

 (W. Atlantic), laqueatum, p. 10. 



a"". Ribs strong, intervals deeply concave with a few rib- 

 lets ; length 50 mill., about 10 times the diam. (Ja- 

 pan), odangulatum, p. 16. 

 a""'. Ribs rounded, obsolete toward aperture; length 50 

 mill, about 12^ times the diam. (Papua), 



lessoni, p. 8. 

 V. Ten to thirteen ribs at and near apex, sometims obsolete near 

 aperture. 



a. Surface finely longitudinally striated on interstices and 

 ribs. 



b. 9-11 wide ribs, subobsolete toward aperture ; 

 length 45 mill., about 7 J times the diam., 



laqueatum, p. 10. 

 b'. 11-12 distinct ribs, intervals and some ribs lin- 

 eated ; length 37 mill., 9 times the diam., 



aratorum, p. 10. 

 b". 13 strong, sharp ribs, without interstitial riblets ; 

 surfece finely striated longitudinally ; length IS- 

 IS mill., 6 times the diam., strigatum, p. 13. 

 a'. Surface not longitudinally striated, interstitial strise few 

 or none. 



b. 10 rather sharp ribs, intervals nearly flat, not 

 lirate ; length 20 mill., about 7 times the diam., 



deeemcostatum, p. 8. 



b'. 8-10 ribs, disappearing toward aperture, length 



50 mill., about 12? times the diam., lessoni, p. 8. 



b". 11-13 strong, narrow ribs, intervals with a weak 



thread or none; length 70 mill, 11-12 times the 



diam., bisexangxdatam, p. 15. 



