190 MOLLITSCA FROM THE CRAG. 



3. Dentalium bifissum. $. Wood. Tab. XX, fig. 3, a — b. 



DiTfiUPA polita. S. Wood. Catalogue in Ann. and Mag. of Nat. Hist. 1842, p. 459, pi. 5, fig. 14. 



D. Testa tereti, leviter arcuatd, subulatd, laevigata, glabra, polita ; ant ice simplici, 

 margine acuta; poslice bifida in utroque latere fissd. 



Shell tubular, tapering, and elegantly curved, smooth, and glossy ; anterior 

 extremity simple, not contracted ; margin acute, posterior termination bilaterally 

 cleft, with the dorsal portion of the margin projecting and rounded, ventral portion 

 short and truncate. 



Length, \ an inch. 



Localitg. Cor. Crag, Sutton. 



Small and imperfect specimens of this shell are abundant in the Coralline Crag. 

 In my Catalogue it was considered as a Pteropod, in consequence of my longest and 

 largest specimen appearing to have a slight contraction at the anterior extremity, 

 which, upon furtheu examination, I believe to have arisen from a portion of the 

 exterior having peeled off at that part, and as there is no one specimen in my 

 collection that can be said to have a contraction at the anterior or larger extremity, 

 it may, perhaps, be more correct to place it in this genus. This species, like 

 D. coarctatum, Lam., has the posterior opening laterally cleft, but not very deeply. 

 The portion on the dorsal side of the fissures projects considerably, and is rounded ; 

 on the ventral side the edge is short and truncated. It differs from D. coarctatum 

 in having this projecting terminal portion rather longer, and in the anterior part of 

 the shell not being contracted. In this shell also the lines of growth are distinctly 

 visible, and rather oblique to the axis of the shell ; in the Grignon specimens of 

 1). coarctatum these lines are not visible under a common lens. The diameter of 

 the posterior opening is very variable, though generally less than half that of the 

 anterior. 



I have no specimens without this peculiar termination, except where evidently 

 broken off. I am, nevertheless, inclined to think that these clefts are produced by 

 the animal at a certain stage of its existence, and that this peculiar form is not the 

 state of the infant shell, as the lines of growth may be seen to cross the fissures, 

 but that, like the young of the preceding species, the posterior termination was. 

 small and circular. 



