DENTALIUM-FISSIDENTALIUM. 63 



This is a very characteristic species, in which the longitudinal 

 sculpture, and even the shell, are often somewhat spirally twisted as 

 much as one-eighth of the circumference. (Dalfy. 



There are at first 9 narrow, acute ribs, with smooth, wide concave 

 intervals; then 3, 4 or more narrow riblets appear in each interval 

 and the section of the tube becomes circular. Toward the aperture, 

 which is slightly compressed laterally, there are subequal, fine and 

 inconspicuous riblets. The slit is rather long and on the concave 

 side. 



Subgenus FISSIDENTALIUM Fischer, 1885. 



Fissidentalium FISCHER, Manuel de Conchyliologie, p. 894 (1885), 

 type D. ergastieum Fisch. 



Schizodentalium SOWERBY, Proc. Malac. Soc. Lond., i, p. 158 

 (1894), type S. plurifissuratum Sowb. 



Shell large and solid, sculptured with many longitudinal riblets, 

 the apex typically with a long slit, but often simple, sometimes with 

 a slit divided into a series of fissures. 



Mainly deep water species, of all temperate and tropical seas, 

 distinguished chiefly by the large size and solidity of the shell with 

 numerous longitudinal riblets. The apical slit is a frequent but 

 by no means invariable feature, being here an extremely mutable 

 character, as in most other groups of the genus. It is this great var- 

 iability, not only between different species but among the individ- 

 uals of the same species, that has induced us against preconceived 

 opinions to merge Mr. Sowerby's group Schizodentalium into' 

 Fischer's earlier subgenus. The extraordinary character of a slit 

 divided into a series of fissures might well induce any conchologist 

 to found a new genus ; but the essential agreement of the type species 

 with Fissidentalium in sculptural characters and contour, the vari- 

 ation in number of the fissures, and the existence of the same char- 

 acter to some degree in other species (capillosum and exuberans), all 

 seem to us to indicate the minor importance of this modification of 

 the slit, in common with the other several types of apical structure. 

 In this case, as throughout the Scaphopoda, data upon the soft anat- 

 omy are required. 



Wide as is the distribution of species of this group in modern seas, 

 the range in time is not less marked. Characteristic fossil forms 

 are D. grande Desh. of the Paris Basin Lower Eocene, a species not 

 unlike D. capillosum; D. giganteum Sowb. and D. corrugatum Gay 



