6 DENTALIUM. 



V. Smaller shells ; length 15 to 35 mill., 10-14 

 times the diam. 



c. West American, neohexagonum, p. 19. 

 c'. West Indian, gouldii, p. 20. 



b". Apical orifice contracted at the sides, 



picteti, p. 22. 

 a'. 12 equal or subequal ribs at aperture, 



intercalation, p. 23 ; duodecimcostatum, p. 13. 



a". Each interval divided and subdivided by successively 



arising riblets, pseudosexagonum, p. 23 ; oerstedi, p. 24. 



II. Seven ribbed at and near the apex. 



a. 14 ribs from aperture to middle ; length 14 mill., 7 

 times the diam., katowense, p. 9. 



a'. Interstitial riblets toward the round aperture ; length 

 14 mill., 10 times the diam. ; a minute apical slit, 



cheverti, p. 9. 



a". 7-8 thick ribs and interstitial threads ; length 72 mill., 

 12 times the diam., japonicum, p. 17. 



a'". 6-7 ribs, interstices smooth or becoming striate toward 

 aperture; length 50-55 mill.. 12-14 times the diarn., 



hexagonum, p. 18. 



III. Eight ribbed at and near the apex. 



a. Ribs strong and very prominent ; interstices deeply con- 

 cave, with some riblets in adult shells. 



b. 7-8 thick ribs; length 72 mill., about 12 times 

 the diam., japonicum, p. 17. 



b'. 8-9 strong ribs ; length 50 mill., about 10 times 

 the diam., octangulatum, p. 16. 



b". 8 strong ribs ; length 10 mill., about 7 times the 

 diam., tasmaniensis, p. 9. 



a'. Ribs narrower, less prominent ; interstitial riblets want- 

 ing or very weak. 



b. Pale tawny or greenish ; intervals wide, flat ; a 

 small apical slit ; length 48-77 mill., 11-13 times 

 the diam., javanum, p. 4. 



b'. White, slender, thin, ribs narrow, interstices 

 transversely striated ; length 65 mill., 13 times 

 the diam., filosum, p. 13. 



b". 8-9 equal rounded ribs, intervals unequal, shal- 

 low, with some threads ; length 30 mill., 8 times 

 the diam., yokohamense, p. 16. 



