31 



Cephalophor'a. 



/ 



5, Dentalium agile. M. Sars. 



n. sp. 

 (PI. III. %. 415.) 



Dentalium incertum. Philippi fauna Molluscorum regni utriusque Sicilian Vol. 2. p. 207 

 (non Deshayes). 



This species, living in the great depths of our seas, for the identification of which with 

 the fossile Dentalium incertum Philippi (non Deshayes) found in the tertiary formation in 

 Calabria, I am indebted to Jeffreys, is certainly very closely allied to I), abyssorum M. Sars; 

 and is perlaps also, as Jeffreys thinks, properly to be considered as only a peculiar deep-sea 

 variety of the same. However I note it here preliminarily as a distinct species, under the 

 specific denomination assigned to it by my Father, since as will appear in the sequel, it shews 

 not a few differences from the ordinary D. abyssorum. If it is really a variety of this latter, 

 the variation is in any case so considerable as to affect the idea of species. To draw a sharp 

 boundary line between what is to be understood by a strongly marked constant variety, and 

 by a species, is moreover --at least from the stand-point of the Darwinian theory, no easy 

 matter; for, according to this theory, the species have had their origin precisely in such va- 

 rieties. It seems to me also that we might with nearly as good reason include in the same 

 category the typical D. entalis L. which in nearly all respects, excepting the striation of the 

 shell, coincides with the D. abyssorum, and consider all these 3 forms as originally belonging 

 together, or in a comparatively new time developed into independent forms (species), in which 

 case the deep-sea species here described would probably be best entitled to rank as the 

 proper parent form. 



The shell (Fig. 4 6. tig. 10) which in the largest specimens attains the considerable 

 length of 58 mill, is immediately distinguished from that of the I), abyssorum by its much 

 more slender, and less curved shape. Sometimes -- and this is, contrarily to the rule, espe- 

 cially the case in larger exemplars (see fig. 4), it is nearly quite straight with only an exces- 

 sively slightly marked ventral curve. Where the curve is greater (see fig. 6), it is however 

 quite uniform; while in the D. abyssorum the proper curve of the shell begins only towards 

 the posterior extremity. Neither does the shell taper so strongly and suddenly towards the 

 posterior end as in D. abyssorum, but always quite evenly from the anterior to the posterior 

 extremity. The whole shell has therefore also a more regular cylindro-coriical shape than 

 that of the other Dentalia. To facilitate comparison, the measurements of several specimens 

 of all our 3 species of Dentalia are here given. 



