DENTALIUM. 7 



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), 



yokohamense, 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 7 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), octangulatum, 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 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 ; 



surface finely striated longitudinally ; length 15- 



18 mill., 6 times the diam., strigalum, p. 13. 



a'. Surface not longitudinally striated, interstitial strise few 



or none. 



6. 10 rather sharp ribs, intervals nearly flat, not 

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



decemcostatum, 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., bisexangulatum, p. 15. 



