PTYCHODUS. 239 



bourne, Sussex; Wantage, Berkshire. Zone of Terebratvlina gracilis: Cuxton, 

 Kent; AVhyteleafe, Surre}'. Zone of Holaster planus : Blue Bell Hill, Burham, 

 Kent; Luton, Bedfordshire; Heytesbury, Wiltshire. Zone of Micraster coran- 

 guinum : Gravesend, Kent; Grays, Essex; Brighton, Sussex; Brandon, Suffolk. 

 Undetermined zones : Guildford and Croydon, Surrey; Hertford; Orford, Suffolk. 



5. Ptychodus decurrens, Agassiz. Plates LI, LII; Text-figures 70, 71, 70, 77. 



1752. Dens piscis Ostracionis, F. E. Bruckmann, Acta Phys. Med., vol. ix, p. 116, pi. v, fig. 4. 



1811. Palate of Unknown Fish, J. Parkinson, Organic Remains, vol. iii, pi. xviii, fig. 12. 



1835-39. Ptychodus decurrens, L. Agassiz, Poiss. Foss., Feuill., p. 54 (name only), and vol. iii, p. 154, 



pi. xxv b, figs. 1, 2, 4, 6 — 8 (non figs. 3, 5). 

 1840-45. Ptychodus decurrens, R. Owen, Odontogr., vol. ii, pis. xviii, xix. 

 1850. Ptychodus decurrens, F. Dixon, Geol. Sussex, p. 362, pi. xxx, figs. 7, 8, pi. xxxi, fig. 1, pi. xxxii, 



fig. 5. 

 1850. Ptychodus depressus, F. Dixon, op. cit., p. 363, pi. xxxi, fig. 9. 

 1850. Ptychodus oweni, F. Dixon, op. cit., p. 364, pi. xxxi, fig. 2. 

 1887. Ptychodus decurrens, A. S. Woodward, Quart. Journ. Geol. Soc, vol. xliii, pp. 123 — 130, pi. x, 



figs. 1—10, 13. 

 1889. Ptychodus decurrens, A. S. Woodward, Catal. Foss. Fishes B. M., pt. i, p. 138. 

 1889. Ptychodus oweni, A. S. Woodward, torn, cit., p. 138, pi. v, fig. 8. 

 1889. Ptychodus multistriatus, A. S. Woodward, torn, cit., p. 146, pi. v, figs. 4-- -6. 

 1894. Ptychodus levis, A. S. Woodward, Proc Geol. Assoc, vol. xiii, p. 192, pi. v, figs. 5, 6. 

 1904. Ptychodus decurrens, A S. Woodward, Quart. Journ. Geol. Soc, vol. Ix, p. 133, pi. xv. 

 1911. Ptychodus decurrens, G. E. Dibley, Quart. Journ. Geol. Soc, vol. lxvii, p. 264, pis. xvii — xix. 



Type. — Detached teeth; one in British Museum. 



Specific Characters. — A species usually of small or moderate size, but the lower 

 median teeth occasionally measuring 6 cm. in transverse diameter. Median 

 portion of tooth flattened or gently raised and crossed by numerous, regular, fine 

 transverse ridges, which usually pass directly and merge into the finely-marked 

 marginal area, but are occasionally recurved into loops in some of the principal 

 teeth (var. multistriatus), and sometimes become irregular (var. oweni). Marginal 

 area generally narrow, and its fine markings more or less radiating or directed at 

 right-angles to the border, rarely in part concentric. Transverse ridges in lower 

 median teeth from 10 to 1G (rarely more) in number. 



Description of Specimens. — The type specimens are detached teeth from the 

 English Chalk, of which only one (the original of Agassiz, pi. xxv b, fig. 8) can 

 now be identified in the Mantell Collection. The species, however, is known In- 

 numerous groups of associated teeth, of which a few are arranged in the matrix 

 in their natural order. These groups exhibit considerable variations not only in 

 different parts of the mouth but also in different individuals ; and some forms of 

 the teeth appear to be worthy of varietal names. 



The typical form of the dentition is shown of the natural size in Text-fig. 70, 



