HYBODUS. 13 



stout and not much arched, with sHo-htlv rounded sides and bhmt anterior keel; 

 lateral face of exserted portion covered with coarse rounded longitudinal ridges, 

 which are closely arranged and in the distal portion tend to become subdivided 

 into tuljercles. 



Bescrijition of Specimens. — Like most of the fossils from the Wealden of Tilgate, 

 the only two known specimens of this form of dorsal fin-spine are much water- 

 worn and abraded. The large spine figured by Mantell (figure copied by Agassiz) 

 is especially abraded, so that traces of the rounded longitudinal ridges (not shown 

 in the published figure) are observable only in the distal half near the front 

 border. The smoothness of the specimen and the bluntness of the posterior 

 denticles are due entirely to abrasion. The second specimen (PL III, fig. 8) is 

 part of the distal half of a spine with the ridged ornament better preserved, and 

 interesting as exhibiting a tendency to the subdivision of the ridges into 

 tubercles. 



There is considerable resemblance between this form of spine and that 

 mentioned above (p. 11) in connection with Hyhodus ensis ; l)ut the longitudinal 

 ridges in the latter are very i^arely nodulose and still more rarely sul)divided. 



IIovi::on and Localifi/. — Tunbridge Wells Sands : Tilo-ate Forest. 



5. Hybodus strictus, Agassiz. Plate III, figs. 4, 5. 



1837. Hijhodus slrictus, L. Agass'.z, Poiss. Foss., vol. iii, p. 45, pi. x, figs. 7 — 9. 



Tj/2)e. — Dorsal fin-spine ; Bristol Museum. 



Specific Characters. — Dorsal fin-spines attaining a length of about 12 or 

 13 cm., slender and not much arched, laterally compresse:!, with a sharp 

 anterior keel ; lateral face of exserted portion covered with sharp, strong, longi- 

 tudinal ridges, well spaced except near the lower part of the posterior border, 

 where they are finer and crowded; posterior denticles moderately large ; inserted 

 base slender and tapering. 



Description of Speciriieus. — The two examples of this fin-spine shown in PL III, 

 figs. 4, 5, are typical, and its characters are very constant in the nuraerousknown 

 specimens. It is of the same general form as the spines named II. snhcariiiafns 

 and H. hasmins, but is distinguished by its stronger ribs and larger posterior 

 denticles. 



Horizon and Localiti/. — Middle Purbeck Beds : Swauage. 



6. Hybodont Cephalic Spines. Plate I, figs. 3, 4. 



The cephalic spine already described in Hyhodus hasanns (p. 10) is closely 

 similar in shape to that of the typical Hi/hodi(s from the Lower Lias ; and several 



