19 Q MOLLUSCA FROM THE CRAG. 



3. DENTALIUM BIFISSUM. S. Wood. Tab. XX, fig. 3, a b. 



DITBUPA POLITA. 5. Wood. Catalogue in Ann. and Mag. of Nat. Hist. 1842, p. 459, pi. 5, fig. 14. 



D. Testa tereti, leviter arcuatd, subulatd, lavigatd, glabrd, politd ; antice simplici, 

 maraine acutd ; postice bifidd in utroque laterejissd. 



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. 



Locality. Cor. Crag, Button. 



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 further 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 \\\ 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 D. coarctatum these lines are not visible under 

 a common lens. The diameter of the posterior opening is very variable, though gene- 

 rally 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. 



