HYBODUS. 11 



2. Hybodus ensis, sp. nov. Plate II, figs. 2 — 7. 



Type. — Tooth ; British Museum, 



tSpecific Characters. — Teeth sometimes 2 cm. in diameter, longest usually 

 smaller. Median cone high, much compressed, broad at the base, tapering 

 gradually to a blunt apex ; two or three lateral cones, slender and sharply 

 pointed, close to the median cone ; coronal surface marked at the base with 

 numerous delicate vertical wrinkles, which nearly reach the apices of the lateral 

 cones. 



Description of Specimens. — This species is definitely known only by isolated 

 teeth, of which the original of PI. II, fig. G, may be regarded as the type 

 specimen. Here the median cone is complete, except for slight abrasion of its 

 apex ; the characteristic slender inner lateral cone is also well shown ; and there 

 is a trace of a minute outer cone on one side. Near the base the fine vertical 

 wrinkles are conspicuous, and they do not extend quite to the apex of the inner 

 lateral cones. The original of fig. 3 is a crushed larger tooth of nearly similar 

 form, with the left lateral cone broken away at the apex. A still larger tooth, 

 nnicli abraded, with imperfect lateral cones, is shown in fig. 5. In the tooth 

 represented in fig. 2 the apex of the median cone is l)lunted by fracture, while in 

 the original of fig. -1 it is complete. Both these teeth have a minute outer pair of 

 lateral cones. Fig. 7 shows a smaller tooth with the median cone much inclined 

 backwards, evidently referable to the hinder part of the jaw. It has three lateral 

 cones in front. In all tl*e teeth the compression of the median cone causes its 

 lateral borders to be especially thin. 



The teeth now described have sometimes been referred to the typically Lower 

 Oolitic species, Hybodus grossicoiius, Ag., but most of them are of smaller size, and 

 they are readily distinguished by the less lateral expansion of their base-line and 

 the somewhat blunter apex of their median cone. 



Dorsal Fin-spines. — It is interesting to notice that in the same horizon as the 

 teeth of Hybodus ensis there also occur dorsal fin-spines almost identical with 

 those named H. dorsalis (L. Agassiz, Poiss. Foss., vol. iii, 18:37, p. 42, pi, x, 

 fig. 1), which are found in Bathonian formations with the teeth of H. (/rossiconus. 

 These spines, of which three are shown in PI. Ill, figs. 1 — o, may therefore 

 possibly belong to H. ensis. They are rather stout, with coarser and rounder 

 ridges than the other Hybodont fin-spines met with in Purbeck and Wealden 

 deposits, and their posterior denticles are relatively large. The crushed specimen 

 represented in fig. 1 is short and wide, with regular smooth ribbing, but only 

 traces of the posterior denticles. The original of fig. 2 is an abraded fragment 

 with very coarse and partly nodulose or wavy ridging. Fig. 3 represents a 

 smaller and more elongated spine with large, irregular, hooked posterior 

 denticles. 



