56 J. S. GARDNER ON THE CRETACEOUS DENTALIID.^. 



5. On the Cretaceous Dentaliid^:. By J. S. Gardner, Esq., F.G.S. 

 (Read June 20, 1877.) 

 [Plate III.] 

 In this paper are described and grouped the shells of the family 

 DentaliidaB which have been obtained from the Cretaceous rocks 

 of Great Britain. It includes all the forms I know of ; but there may 

 be in local museums and private collections specimens I have not seen. 



In the genus Dentalium I have included all the forms belonging 

 to this family which are not fissured at the apex, and those which 

 are not swollen or constricted at the aperture. 



The Cretaceous species here described, included in the genus 

 Dentalium thus constituted, may for the present be arranged under 

 two subdivisions. 



A. Dentalium proper, comprising the larger forms, all of which 

 are striated. 



B. The smaller and smooth forms. 



In the first of these groups four species are described : two of these 

 are new ; and two, D. decussatum and D. medium, which were given 

 by Sowerby in the ' Mineral Conchology,' are redescribed, as it seemed 

 to me that his descriptions are incomplete. My description of 

 D. decussatum is based on more than 50 specimens in my collection ; 

 and with it I have been able to incorporate D. striatum and D. 

 ellipticum of Sowerby, as they are not distinct species. The latter 

 was regarded by D'Orbigny as a synonym of D. decussatum. The de- 

 scription of D. medium is based upon specimens in the British 

 Museum, the Jermyn-Street Museum, and Mr. Meyer's collection. 

 Sowerby's figured specimen is evidently an internal cast, in which, 

 of course, no trace of external markings was preserved : but a perfect 

 specimen is figured, Trans. Geol. Soc. vol. iv. pi. xviii. p. 343. Much 

 confusion has arisen, owing to the imperfection of Sowerby's original 

 figure and to an unfortunate mistake in putting together in his 

 collection both smooth and striated Dentalia from Blackdown. Of 

 the two new species, I have named a very long and slender shell, 

 like some of the recent Dentalia in aspect, D. divisiense ; D. alatum, 

 the other form, demands special notice, as it possesses two lateral 

 ledge-like expansions, the use of which can only be guessed. There 

 are species, however, among the Cretaceous Dentalia, much thickened 

 laterally, which seem to lead up to this form, in which the thicken- 

 ing culminates. D. decussatum seems to show us that some of these 

 Gault Dentalia had the power of thickening their shells generally to 

 an almost indefinite extent. The rugose and uneven appearance 

 presented by some seems to indicate that individuals among them 

 lived to a great age. Besides these species, Dr. Fleming long since 

 described one under the name of D. septangular e *, which I have 

 never seen ; and since the reading of the present paper I have 

 described a new species from the Grey Chalk of Dover under the 

 name of D, majus f . 



* Edinb. Phil. Journ. xii. pi. 9. 

 t Geol. Mag. n. s. vol. iv, p. 556. 



