HYBODUS. 



9 



overturned and displaced. Each is shown to have been supported in tlie usual 

 manner by a dorsal tin-spine fixed to a triangular basal cartilage, which extends 

 from the inserted end of the spine throughout the Avhole length of the fissure on 

 its hinder face. At the distal border of the basal cartilage of the posterior fin five 

 small radials also occur, gradually increasing in length towards the hinder edge of 

 the fin ; and there are traces of delicate filiform rays for the support of the fin- 

 membrane. As in Hi/J)()Jiis fraasi and //. liauffianus, the basal cartilage of tlie 

 anterior fin is narrower and deeper than that of the posterior fin. 



The dorsal fin-spines are much laterally compressed and very little arched, 

 with a comparatively slender base of insertion. The sides of the exserted portion 



Fig. 5. — Hyhodus hasmius, E<jerton ; fragment of trunk in left side view, showing the neural arches 

 (n.s.) of the vertebral axis, with the spines and cartilages of the anterior {d. 1) and posterior (d. 2) 

 dorsal fins, the latter overturned and displaced, nearly one-half nat. size. —Weald Clay : Pevensey 

 Bay, Sussex. Beckles Collection (B. M. no. P. 6357). 



are completely covered with fine and sharp longitudinal ridges, which are 

 sometimes slightly nodulose where crossed by growth-lines. Near the base about 

 eight ridges are widely spaced, while four or five at the posterior border are 

 crowded. The posterior denticles are numerous, small, and closely arranged. As 

 shown in Text-fig. 5, the spine of the anterior fin is broader than that of the 

 posterior fin. 



In some specimens, which are probably to be regarded as males, there is also 

 a single pair of spines immediately behind the head. This is best shown in the 

 partially decayed skull represented in PI. I, fig. 2. The spine (.s-.) is placed 



