PTYCHODUS. 23] 



In another typical specimen, the teeth are considerably smaller and exhibit a 

 more pronounced elevation of their median portion, at least in the upper jaw 

 (PI. XL VII, figs. 19, 20). A small tooth of the upper middle row (fig. 19, o') is 

 nearly heart-shaped, and its conspicuous median raised portion, though now worn, 

 must have been crossed originally by four or five well-marked ridges. A tooth 

 which seems to be referable to the inner or second paired row of the upper jaw 

 (fig. 20) is clearly unsymmetrical, and the outer paired teeth (fig. 19) show the 

 usual oblique distortion and the predominance of the marginal area in their outer 

 half. In other specimens the lateral extension or widening of the outer paired 

 teeth is still more marked than in those now described. 



In a few teeth with the marginal granulation still fine and typical, the median 

 portion is less elevated and more coarsely ridged (PI. XLVII, figs. 21, 22). This 

 condition, however, is commoner in teeth with a coarsely granulated marginal 

 area, which are known from several localities but cannot yet be recognised as 

 characterising any particular zone. A tooth from Brighton figured by Dixon 

 (<>p. cif., pi. xxx, fig. 6) is normal in all respects except this coarseness of the 

 surface-markings. In some associated teeth from the Mantell Collection 

 (PI. XLVII, figs. 23 — 27), the median raised portion is less elevated, and is 

 marked off from the marginal area by a more or less well-defined groove. 



In the teeth of P. mammillaris the transverse ridges are scarcely ever sub- 

 divided or irregular. One specimen in the Coombe Collection (B.M. no. P. o -391) 

 exhibits a few tubercles intercalated between some of the ridges. 



Horizons and Localities. — Turonian zones: near Lewes and Brighton, Sussex. 

 Zone of Bhynchonella cuvieri: Burham, Kent; Betchworth and Dorking, Surrey; 

 Culver Cliff, Isle of Wight; Pinhay Cliff, Dorset. Zone of Terebratulina gracilis : 

 Hailing, Kent. Zone of H<Haster planus .- Cuxton, Kent; Clothall, Hertfordshire; 

 Luton, Bedfordshire; Culver Cliff, Isle of Wight; Heytesbury, Wiltshire. Zone of 

 Mi <'faxter cor-testudinarium : Beer, Devon. Undetermined zones: Guildford, 

 Surrey; West Harnham,Wiltshire. 



2. Ptychodus rugosus, Dixon. Plate XLVIII, figs. 1—11. 



1850. Ptychodus rugosus, F. Dixon, G-eol. Sussex, p. 362, pi. xxxi, fig. 5. 



1850. Ptychodus alitor, F. Dixon (en-ore), oj>. cit., p. 362, pi. xxx, fig. 10. 



1889. Ptychodus rugosus, A. S. Woodward, Catal. Foss. Fishes B. M., pt. i, p. 136, pi. v, figs. 1—3. 



1911. Ptychodus rugosus, Gr. E. Dibley, Quart. Journ. Geol. Soc, vol. lxvii, p. 272, pi. xxii, fig. 6. 



Type. — Detached tooth. 



Specific Characters. — Teeth as in P. mammilla ris, but the summit usually more 

 rounded, all the surface-markings less well-defined, and the transverse ridges 

 more or less irregular or interrupted. 



