HYL.EOBATIS. 



21 



A closely similar arrangement is seen in I'tijcliodus (Text-fig. 11). Another trans- 

 versely-elongated tooth (PI. V, fig. 5) has been so ninch worn during life that most 

 of its surface markings have been removed ; but it is interesting as showing a 

 distinct bevelling at one end and a single well-marked pressure-scar at the other. 

 The original of PI. V, fig. 4, which is a less elongated tooth, has also been worn 

 during life, but its oral face remains convex and seems to have been opposed to 

 two teeth of a transverse series in the mouth. One lateral end of this tooth is 

 strongly bevelled for articulation with two teeth. 



IIorizo)i and Lor(dities. — Weald en : Brook, Isle of Wight; Sevenoaks, Kent. 



Fig. 11. — Ptychodus mammillaris, Agassiz ; vertical transverse section of crown of tootli,liig'lily niagni- 



lieil. — Ensflish Chalk. After Ao-assiz. 



Subclass TELEOSTOMI. 

 Order CnCSSOPTER YGII. 



Family CcEr.Ao.^NTHin.E. 



Genus UNDINA, Minister. 



Undina, G. vou Miinster, Neues Jahrb. f. Min., etc., 1834, p. 539. 



Hnlopliagus, P. M. G. Egerton, Figs, and Desciipts. Brit. Oiganic Remains, dec. x (Mem Geol. Surv., 

 1861), p. 19. 



Generic Gharader.s. — External bones and scales superficially ornamented with 

 tubercles or fine interrupted ridges of ganoine ; parafrontal and circumorbital bones 

 ])late-like, without superficial excavations. Teeth absent on the margin of the 

 jaws, but a few hollow conical teeth within. Supplementary caudal fin prominent ; 

 the rays of all the fins robust, often expanded, and Avith numerous articulations in 

 the distal portion ; small upwardly-pointing denticles on the preaxial rays of the 

 first dorsal and caudal fins. 



