104 A. Smith Woodward — Acrodus Fost-Liassic. 



the teeth of D, canaliculatus ; nor is it possible, upon such fragmentary- 

 materials, to formulate any very definite specific diagnosis. The A. 

 polydyctios of Eeuss is evidently rather smaller, and may be distinct ; 

 but the figure of the tooth made known by Dixon as A. cretaceus 

 does not suffice to distinguish it from Egerton's fossil already men- 

 tioned and I have not seen the type. 



A. Illingworthi, Dixon (" Geol. and Foss. Sussex," 1st ed. p. 364, 

 pi. XXX. figs. 11, 12, and pi. xxxii. fig. 9.) 



From an examination of the detached teeth of this species, it is 

 impossible to determine whether it is to be referred to the generic 

 type under consideration, or ought rather to be placed with those 

 Cretaceous teeth hitherto regarded as pertaining to Hyhodus. The 

 National Collection comprises a small group of about twelve naturally 

 associated examples, but this does not afford any information as to 

 the character of the complete dentition ; and the discovery of further 

 specimens must be awaited before it is possible to arrive at any 

 definite conclusions. In form, the English fossils are remarkably 

 similar to some minute teeth described by Eeuss from the Chalk of 

 Bohemia, under the name of Hyhodus folyptyclms} 



As already mentioned, comparatively satisfactory remains of at 

 least one post-Liassic Selachian, with teeth of the ^cro(^MS-type, have 

 been described by Wagner (Loc. cit.) from the Lithographic Stone (or 

 Lower Kimmeridgian) of Solenhofen, under the name o^ A. falcifer. 

 This species, however, differs so much from the typical forms of 

 Liassic age, that it seems not unlikely that future discoveries may 

 lead to its generic separation ; the dentition, for example, is less 

 uniform, the symphysial teeth being conical and pointed ; the dorsal 

 spines and shagreen are quite different, and there are no dermal 

 " SfhenoncM " in the cephalic region ; and there is considerable reason 

 for suspecting that the vertebral column attains a much higher stage 

 of development. 



It seems also very probable that some of the species described 

 above, upon mere dental evidence, may eventually prove to be equally 

 distinct. As regards the Jurassic and Wealden forms, of course, it is 

 quite possible that they were Sharks of the ordinary type as repre- 

 sented in the Lias ; for both ribbed dorsal spines and dermal booklets 

 (SpJienoncM) are not unfrequently met with in the same deposits. 

 But since none of these appendages can be distinguished from those 

 of the true Hyhodus, and since also there are numerous Hybodont 

 teeth upon the same horizons, it is equally probable that they all 

 pertain to the latter. 



In Cretaceous formations, however, dorsal spines with a ribbed 

 ornamentation are almost entirely wanting ; Owen's Hyhodus com- 

 planatus,^ in fact, appears to be the only example hitherto described, 



1 A. E. Eeuss, " Verstein. bohm. Kreideform." pt. ii. p. 97, pi. xxi. figs. 9, 10. 



^ R. Owen, "Notes on Two Ichthyodorulites hitherto undescribed," Geol. Mag. 

 Vol. VI. 1869, p. 482. The small fragments in Dr. Mantell's Collection, said to 

 have been obtained from the Chalk of Lewes, and described by Agassiz (op. cit. vol. 

 iii. p. 44, pi. 103, figs. 15, 16) as Hyhodus sulcalus, are undoubtedly Wealden 

 fossils : see S. J. Mackie, "The Geologist," vol. vi. p. 242. 



