14 WEALDEN AND PURBECK FOSSIL FISHES. 



portions of spines found isolated both in Wealden and Parbeck formations agree 

 with these in their striated ornament and terminal barb. Two fragments, obtained 

 by Mantell from the Tnnbridge Wells Sands of Tilgate Forest and apparently 

 worn smooth by abrasion, were described under the name of Splienonchus elongatus 

 by Agassiz, Poiss. Foss., vol. iii, 1843, p. 202, pi. xxii a, figs. 18, 19. Besides these 

 typical cephalic spines, however, there also occur in Wealden beds comparatively 

 small specimens in which the enamelled exserted portion is reduced to a smooth 

 pointed hook without any barb. One is well seen in side view in PI. I, fig. 3, and 

 a nearly complete example is shown both in side and outer view in PI. I, figs. 4, 4«. 

 Here the inserted triradiate base is remarkably large, with its postero-inferior 

 limb small and the two lateral limbs much enlarged and inclined downwards. 

 This, indeed, seems to represent the final degenerate condition of the Hybodont 

 cephalic spine. 



Genus ACRODUS, Agassiz. 



Acrodus, L. Agassiz, Poiss. Foss., vol. iii, 1838, p. 139. 



Sphenonchus, L. Agassiz, op. cit., vol. iii, 1843, p. 201 (in part). 



Thectodus, H. vou Mevcr and T. Plieninger, Beitr. Paliiont. Wiirtembergs, 1844, p. 116. 



Generic Character)^. — Only differing from IIi/ho<his in the rounded, non-cuspidate 

 shape of the teeth. 



Ti/2)e Species. — The generic name appears to have been given by Agassiz first 

 to the teeth of Acrudus gaillardoti from the German Muschelkalk, in Gaillardot, 

 Ann. Sci. Nat., ser. 2, vol. iii (Zoologie), ] 835, p. 49, and in Mougeot, Bull. Soc. 

 Geol. France, vol. vi, 1835, p. 20, but it was not defined until the discovery and 

 description of Acrodus nohilis from the Lower Lias of Lyme Regis (Agassiz, Poiss. 

 Foss., vol. iii, 1838, p. 140, pi. xxi). The latter must therefore be regarded as 

 the type species. 



ReniarJcs. — Acrodus is best known by the remains of ^1. nuhilis: and ^4. amiinqiai 

 from the Lower Lias of Lyme Regis, described in Catal. Foss. Fishes, Brit. Mus., 

 pt. i, 1889, pp. 283-295, pis. xiii, xiv. The arrangement of the dentition is shown 

 in Text-fig. G. 



1. Acrodus ornatus, A. S. Woodward. Plate II, figs. 15 — 18. 



1889. Acrodus ornatiix, A. S. Woodward, Catal. Foss. Fishes, Brit. Mus., pt. i, p. 296, pi. xiii, fig. 10. 



Tijije. — Detached tooth ; British Museum. 



Specific Characters. — A very small species known only by detached teeth, 

 which do not exceed about 7 mm. in length. The dental coronal contour is low 



