244 FOSSIL FISHES OF THE ENGLISH CHALK. 



obliquely distorted. The average transverse measurements (in millimetres) of the 

 several rows preserved are as follows: o', 7; l', 17; n', 12; in', 9; iv', 7. 



In another set of scattered but associated teeth a lower median tooth is 

 normal (PI. LI, fig. 3), while the lateral teetli (figs. 1, 2) exhibit more or less 

 distinct gyrations surrounded by an area of granulations which form concentric 

 rather than radiating lines. In a third set discovered and described by Mr. 

 Dibley {Joe. cit., 1911), the ridges on the lower median teeth are partly gyrate, 

 partly irregular (PI. LII, figs. 12, 13), while those of the lower paired rows are 

 normal (fig. 14), and a tooth of the upper inner paired row (fig. 15) begins to 

 make some approach to the corresponding tooth of var. oweni. 



The extreme gyration of the transverse ridges occurs in the depressed teeth 

 which have been described under the name of P. multistriatus (A. S. Woodward, 

 he. cit., 1889). A lower median tooth of the type set, which shows well-marked 

 gyrations in its hinder half, is crossed by no less than 20 ridges. In an associated 

 short and broad tooth the ridges are nearly normal, while in a smaller lateral 

 tooth gyrations are again conspicuous. Another tooth of the same set, which 

 seems to belong to the upper jaw, exhibits a gyration anteriorly. 



P. decurrens is the only species of which teeth have been found in the English 

 Chalk in their natural order. It is also the only species of which the jaw- 

 cartilages and vertebral centra have been discovered in association with the teeth. 

 These skeletal parts have already been described above, pp. 226, 228. 



Horizons and Localities. — Turonian zones : neighbourhood of Lewes and 

 Brighton, Sussex. Zone of Holaster subglohosus: Hailing, Holborough, Wouldham, 

 Blue Bell Hill, and Dover, Kent; Oxted, Merstham, and Dorking, Surrey; Tring, 

 Hertfordshire; Cherry Hinton, Cambridgeshire; Wnittington, Norfolk; Win- 

 chester. Zone of Bhynchonella cuvieri: Betchworth, Surrey. Zone of Holaster 

 planus: S waff ham, Norfolk. Undetermined zone : Guildford, Surrey. 



6. Ptychodus aff. mortoni, Mantell. PI. LIV, fig. 1. 



(?) 1850. Ptyclwdus mortoni, F. Dixon, Geol. Sussex, p. 364, pi. xxxi, figs. 6, 7. 



1894. Ptychodus mortoni, A. S. Woodward, Proc. G-eol. Assoc, vol. xiii, p. 191, pi. v, fig. 4. 



1911. Ptychodus mortoni, G. E. Dibley, Quart. Journ. Geol. Soc, vol. Ixvii, p. 272, pi. xxii, fig. 8. 



Type (of Ptychodus mortoni, Mantell, in S. G. Morton, Journ. Acad. Nat. Sci. 

 Philad., vol. viii, 1842, p. 215, pi. xi, fig. 7). — Detached tooth from the Upper 

 Cretaceous of Alabama, U.S.A.; British Museum. 



Description of Specimen. — A tooth from the Chalk of Shoreham, which appears 

 to have been lost, was referred to P. mortoni by Dixon, op. cit., but his determina- 

 tion is doubtful. The only other known specimen from the English Chalk which 

 nearly resembles the teeth of the American species, is shown of the natural size 

 from the upper and anterior aspects in PI. LIV, figs. 1, la. It is merely the 



