64 DENTALIUM-FISSIDENTALIUM. 



of the southern extremity of South America, both probably Miocene 

 or later ; D. mantelli Zittel and D. solidum Button of the New Zea- 

 land upper Eocene or lower Miocene, and other species. 

 The following is a very imperfect analysis of the species. 



Key to species. 



I. Shell circular or nearly circular in section. 



a. 12-14 high, rather acute ribs toward apex, with numer- 

 ous riblets developing as the tube enlarges ; shell large, 

 nearly straight ; a long often sinuous fissure. Length 63 

 -114 mill. rectum, p. 81 ; delessertianum, p. 81. 



a'. Longitudinal riblets or ribs much more numerous. 



b. Slit divided into a series of fissures by bridges of 



shell. 



c. Stout; length 95 mill., about 5 times the great- 



est diam. ; off West Africa, exuberans, p. 78. 

 c'. Length 64 mill., about 8-9 times the diam. ; 

 many unequal ribs and striae ; fissures 2 to 5 ; 

 Hong Kong ?, plurifissuratum, p. 82. 



b f . Slit simple or wanting. 



c. Caliber quite rapidly enlarging, the diameter gen- 

 erally contained 4 to 7 times in the length. 

 d. 50-80 riblets on larger part of the very 

 stout shell. 



e. Very large, yellowish, with about 50 

 riblets; aperture oblique, no slit; 

 length 90-99 mill., 5-5 times the 

 greatest diam. ; E. Pacific, 



megathyris, p. 67. 



e'. 70-80 riblets ; a slit; length 42-45 



mill., 4|-6 times the diam. ; Pacific* 



ceras, p. 68. 



e". Nearly straight, brownish-yellow, 

 with a short fissure ; about 50 very 

 slightly raised rounded ridges, faint 

 toward aperture ; length nearly 50 

 mill., about 6J times the diam., 



amphialum,p. 71. 



d'. 30-36 riblets toward larger end, either 

 alternately smaller or only half as many 



