188 MOLLUSCA FROM THE CRAG. 



He has formed one section of this genus containing those species in which the 

 posterior extremity has a medial dorsal cleft, the nature of which is as yet but 

 imperfectly understood; but I imagine this character will prove insufficient for 

 specific determination. 



1. Dentalium costatum. J. Sow. Tab. XX, fig. a—f. 



Dentalium costatum. J. Sow. Min. Conch, t. 70, fig. 8, 1814. 



_ _ s. Wood. Catalogue 1842. 



— — Morris. Catalogue of Brit. Foss. p. 143, 1843. 



— — Nyst. Coq. foss. de Belg. p. 344, pi. 35, fig. 2, 1844. 



— dentalis (?). Desh. Monog. de Dent. p. 33, pi. 2, fig. 9-10, 1825. 

 _ _ Gould. Inv. of Massach. p. 156, fig. 1, 1841. 



— striatum. Mont. Test. Brit. p. 495, 1803. 



Not. Dentalium fossile. Desh. Monog. de Dent. p. 35, pi. 3, fig. 12. 



D. Testa tereti, eleganter arcuatd, subulatd ; longitudinaliter costatd, costis 10 — 18, 

 obtusis, convexis, eminentioribus ; striis transversis minimis ; extremitate posticd, subpro- 

 funde jissd ; juventute acuminata. 



Shell tubular, elegantly arched, and tapering ; longitudinally costated ; costaa 

 obtuse, convex, and prominent, varying in number from ten to eighteen ; finely 

 striated transversely; anterior margin thin and sharp ; posterior extremity with a 

 dorsal cleft, acuminated and open, but not cleft, in the young state. 



Length, If of an inch ; diameter, nearly 2 lines. 



Locality. Cor. Crag, Sutton. 



Red Crag, Sutton. Recent, Mediterranean. 



Fragments or imperfect specimens of this species are abundant in the Coralline 

 Crag, and they present a great variety in the number, magnitude, and arrangement 

 of the costae, which, in some specimens, are broader than the spaces between them, 

 while in others they are narrower. They are always obtuse and rounded, often 

 irregular in distance, and are continued from one extremity to the other, with 

 sometimes a narrower intermediate line. In perfect specimens there is a medial 

 dorsal cleft to the depth of a line ; and in one individual the posterior extremity 

 has not only a slit, but the terminal aperture is partially covered with a convex 

 epiphragm, that has a cleft across it (fig. 1 /) , like the posterior termination of D. 

 Ussura, Sowerby's Genera. In small specimens, of what I presume to be the young 

 of this species, the posterior termination is entire, without the cleft (fig. 1 e) ; it is 

 evident, therefore, that the animal has the power of absorbing or reducing a portion 

 of the younger shell, and of forming the cleft after it has attained a certain size. 

 The anterior margin is thin and sharp, slightly oblique, projecting a little on the 

 ventral side, with the lines of growth at all times visible, giving the appearance of 

 having transverse strige. A recent specimen of D. dentate, from the Mediter- 

 ranean, given me by my friend Professor E. Forbes, corresponds with our shell 

 in its external characters, but it is less in size, and has not the posterior cleft. 



