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. /. Sow. Tab. XX, fig. af. 



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. 



DBNTALIS (?). Desk. 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. Desk. Monog. de Dent. p. 35, pi. 3, fig. 12. 



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

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

 fundejissd; juventute acuminatd. 



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

 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, 1| 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. I/), 

 like the posterior termination of D. fasura, 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 striae. A recent specimen of D. dentate, 

 from the Mediterranean, 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. 



