2:52 FOSSIL FISHES OF THE ENGLISH CHALK. 



Description of Specimens. — The close resemblance of the ordinary teeth of this 

 form to those of the type species is well illustrated by the associated series shown 

 in PI. XLVIII, figs. 1 — 5. These five specimens are from a group of about fifty 

 teeth, all apparently belonging to the upper jaw. A median upper tooth (fig. 1 ) 

 is remarkably smooth and very small. In the comparatively large teeth of the 

 inner paired row (figs. 2, 3), two or three of the transverse ridges are uninter- 

 rupted, but they end suddenly, leaving the sides of the coronal eminence nearly 

 smooth. There are slight traces of radiating grooves crossing the feebly-granu- 

 lated marginal area. The lateral teeth (figs. 4, 5) exhibit the usual oblique 

 distortion and relatively wide outer margin. 



Some smaller teeth evidently belonging to the same species (figs. C>, 7) are 

 characterised by an extremely elevated and laterally compressed central portion, 

 of which the whole periphery is smooth. Those of the inner paired row also 

 exhibit a comparatively narrow granulated marginal area. There are distinct 

 gradations between these and the more normal teeth, and they may belong to 

 immature individuals. 



An apparently associated set of comparatively large teeth (figs. 8 — 11) may 

 be regarded as representing a very old individual. In these teeth the median 

 raised portion is much rounded, and the coarse though feeble surface-markings 

 extend over its sides. Sometimes there is a slight constriction between the raised 

 portion and the marginal area; while in one case (fig. 11) a curious malformation 

 is observable. 



Horizons and Localities. — Zone of Micraster coranguinum : Purley, Surrey; 

 Charlton, Strood, Grillingham, Northfleet, and Greenhithe, Kent. Zone of Micras- 

 ter cor-testudinanum: Croydon, Surrey. Undetermined zones: Guildford, Surrey; 

 Houghton Pit, Arundel, Sussex; Dover, Kent. 



3. Ptychodus polygyrus, Agassiz. Plate XLVIIT, figs. 12—16; Plate XLIX. 



1822. Teeth allied to Biodon, G. A. Mantel], Foss. S. Downs, p. 231, pi. xxxii, figs. 23, 24. 



1835-39. Ptychodus polygyrus, L. Agassiz, Poiss. Foss., Feuill., p. 55 (name only), and vol. iii, 



p. 156, pi. xxv, figs. 4—11, pi. xxvb, figs. 21-23. 

 1837-43. Ptychodus latissimus, L. Agassiz (errore), op. cit., vol. iii, p. 157, pi. xxva, fig. 8. 

 1837. PI ychodus polygyrus, W. Backland, Geol. and Mineral., ed. 2, vol. ii, p. 48, pi. xxvii/. 

 1850. Ptychodus polygyrus, F. Dixon, Geol. Sussex, p. 363, pi. xxx, fig. 9, pi. xxxi, fig. 10. 

 1850. Ptychodus latissimus, F. Dixon (errore), op. cit., pi. xxx, figs. 1, 2. 

 1863. Ptychodus polygyrus, S. J. Mackie, Geologist, vol. vi, p. 161, pi. ix. 



1887. Ptychodus polygyrus, A. S. Woodward, Quart. Journ. Geol. Soc, vol. xliii, p. 127, pi. x, fig. 11. 

 1889-90. Ptychodus polygyrus, A. S. Woodward, Catal. Foss. Fishes B. M., pt, i, p. 143 (non pi. v, 



fig. 7) ; and Ann. Rep. Yorks. Phil. Soc, 1889, p. 40, pi. i, figs. 15—20. 

 1908. Ptychodus polygyrus, G. Sheppard, Naturalist, p. 189, pi. xvi, fig. 14. 

 1911. Ptychodus polygyrus, G. E. Dibley, Quart. Journ. Geol. Soc, vol. lxvii, p. 268, pi. xxi, pi. xxii, 



figs. 1, 2, 9. 



