PTYCHODUS. 235 



ascribed to the upper second paired row (u') are nearly similar, but smaller, with 

 ten to twelve transverse ridges. The more lateral teeth (in'-vi') begin to exhibit 

 transverse elongation and slight oblique distortion, while their median portion is 

 relatively smaller and their transverse ridges are less conspicuously gyrate at the 

 ends. The large lower median teeth (fig. 11, o) are raised as in the typical 

 P. polygyrus, but Avith more regularly gyrate transverse ridges, which are twelve 

 to fourteen in number. The only teeth which seem referable to the lower inner 

 paired row (r) are essentially similar to those of the upper second paired row ; 

 while those of the lower second paired row (n) are either a little wider or a 

 little narrower than those of the third upper paired row. 



A tooth wrongly ascribed to P. latissimus by Dixon (op. cit., pi. xxx, fig. 2) is 

 remarkable for the unusually great and sharp elevation of its median portion. It 

 is crossed by nine transverse ridges and probably belongs to the inner paired row 

 of either the upper or lower jaw. The same appearance of lateral compression, 

 but with a more gently raised crown, is seen in the lower median teeth of an 

 associated set which seems to be referable to var. marginalis (PI. XL VIII, fig. 15). 

 Each of these teeth is deeply impressed at the lateral border with smooth facettes 

 for the adjacent teeth (fig. 15 a) ; and their gently rounded crown is crossed by 

 about thirteen transverse ridges, which extend almost from edge to edge. The 

 teeth of the inner paired row (fig. 16), either upper or lower, are flatter and 

 crossed by eleven transverse ridges, which gyrate into the marginal granulation 

 in the usual manner. The smaller lateral teeth do not exhibit any features worthy 

 of note. 



Horizons ami Localities. — Turonian zones: near Lewes, Sussex; Wantage, 

 Berkshire. Zone of BJwnchonella cuvieri : Dover and Dunton Green, Kent ; 

 Beachy Head, Sussex. Zone of Terebratulina gracilis : Hailing, Blue Bell Hill, 

 Wouldham, Charing, and Lenham, Kent. Zone of Micraster cor-testudiriarium : 

 Dover. Zone of Micraster coranguinum : Banstead and Croydon, Surrey ; Grays, 

 Essex ; Charlton, Bromley, Northfleet, and Gravesend, Kent ; Horsebridge 

 and Micheklever, Hampshire; Amesbury, Wiltshire. Zone of Actinocamax 

 quadratus : Salisbury. Uintacrinus - zone : Salisbury. Undetermined zones: 

 Guildford ; Winchester. 



4. Ptychodus latissimus, Agassiz. Plate L; Text-figures 74, 75. 



1811. Palate of Unknown Fish, J. Parkinson, Organic Remains, vol. iii, p. 262, pi. xix, fig. 18. 



1822. Tooth allied to Diodon, O. A. Mantell, Foss. S. Downs, p. 231, pi. xxxii, fig. 19. 



1835-43. Ptychodus latissimus, L. Agassiz, Poiss. Foss., Fenill., p. 54 (name only), and vol. iii, p. 157, 



pi. xxv a, figs. 1 — 7. 

 1839 — 40. Ptychodus latissimus, E. Owen, Eep. Brit. Assoc, 1838, p. 140; and Odontogr., descr. to 



pi. xvii, figs. 1, 2. 

 1850. Ptychodus paucisulcatus, F. Dixon, Geol. Sussex, p. 363, pi. xxx, fig. 3. 



