292 DENTALIUM. 



shell is acciflentally broken off, the animal can repair it, and 

 in species which have a simply tubular mantle and a thick 

 shell the repairs take the shape of a small tube projecting from 

 the blunt end of the large one, as it is impossible for the mantle 

 to secrete a shell which is as large and thick as the original 

 at the point of truncation." From this it is obvious that in 

 such a thick shell as is possessed by the typical species of 

 Dentalimn, a small portion of the posterior orifice having been, 

 by accident, broken off, it would not be within the animal's 

 power to rehabilitate it in its entirety. 



On the other hand, the possibility of the small notches on the 

 typical specimens having been produced, not by any modification 

 in the mantle of the animal, but by erosion, should be considered. 

 It is well known that erosion, especially in specimens from 

 deep water, modifies and sometimes imitates such slits. As 

 Professor Dall observes,' there "seems to be a peculiarity of 

 some kind in the external prismatic layer of Dentalium, which 

 lends itself to the propagation of erosion in longitudinal lines 

 very much more effectively than at right angles to such lines." 



In regard to the systematic value to be attached to the notch, 

 slit, or fissure, it may be remarked that these variations depend 

 upon the shape of the edge of the mantle, which is an extremely 

 contractile membrane, capable of secreting shelly matter. When 

 the shell is quite perfect, the posterior end reflects the form 

 of the membrane which secreted it. There can be no doubt 

 whatever that the length of the slit varies in individuals of 

 the same species, apart from any appearance produced by erosion. 

 It cannot, however, be asserted that a mere notch could by 

 individual peculiarity be elongated to such a length as obtains 

 in typical specimens of Fustiaria. So that it may now be 

 admitted that the presence or absence or length (within certain 

 limits) of the slit is not of generic, nor even subgeneric importance, 

 but it may with other characters be of specific value. In any 

 case it seems desirable that Entaliopsis { = Entalis) should be 

 regarded as a synonym of Dentalium ; the sole difference between 

 them, as now shown, being merely one of external ornament. 

 In this Mr. E. BuUen IN'ewton fully concurs with the present 

 writer. 



Type. — Dentalium elrphantinum, Linnaeus. 



1 Op. cit. p. 437. 



