230 FOSSIL FISHES OF THE ENGLISH CHALK. 



the typical Myliobatidse of the Tertiary period. Flanking- the median row there 

 are at least three paired rows of teeth, which rapidly diminish in size outwards. 



1. Ptychodus mammillaris, Agassiz. Plate XLVII, figs, lo— 27; Text-figure 72. 



1822. Teeth allied to Biodon, G. A. Mantell, Foss. S. Downs, p. 231, pi. xxxii, figs. 17, 18, 20, 21, 



25, 27, 29. 

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



p. 151, pi. xxvb, figs. 12—20 (? fig. 11). 

 1835-39. Ptychodus altior, L. Agassiz, op. cit., Feuill., p. 54 (name only), and vol. iii, p. 155, 



pi. xxvfc, figs. 9, 10. 

 1839. Ptychodus decurrens, L. Agassiz (errore), op. cit., vol. iii, p. 154, pi. xxvfr, figs. 3, 5. 

 1850. Ptychodus mammillaris, F. Dixon, G-eol. Sussex, p. 361, pi. xxx, fig. 6, pi. xxxi, fig. 4. 

 1850. Ptychodus latissimus, F. Dixon (errore), op. cit., pi. xxxi, fig. 3. 



1889. Ptychodus mammillaris, A. S. Woodward, Catal. Foss. Fishes B.M., pt. i, p. 133. 



1890. Ptychodus mammillaris, A. S. Woodward, Ann. Rep. Yorks. Phil. Soc, 1889, p. 39, pi. i, 

 figs. 3—14. 



1911. Ptychodus mammillaris, G. E. Dibley, Quart. Journ. G-eol. Soc, vol. lxvii, p. 271, pi. xxii, 

 figs. 3—5. 



Type. — Detached teeth ; British Museum, and National Museum of Natural 

 History, Paris. 



Specific Characters. — The type species, of which the largest known teeth 

 measure about 4"5 cm. in transverse diameter. Median portion of tooth much 

 raised, and more or less sharply defined from the granulated marginal area; its 

 upper surface flattened and crossed by regular, prominent, transverse ridges, which 

 pass down the sides and are often continued gradually into the concentric lines of 

 well-marked marginal granulations. Marginal area relatively wide, covered with 

 granulations which are mostly elongated and arranged in lines concentric with the 

 border of the crown ; this area also often crossed by a few shallow radiating grooves. 



Description of Specimens, — In the more typical teeth of this species the 

 median raised portion gradually passes into the marginal area without any 

 nterposed groove, while its regular transverse ridges usually end in coarse 

 tubercles which soon become finer and tend to be arranged in lines continuous 

 with the concentric rows of granulations on the marginal area. The granulations 

 are often so fine that they give the border of the tooth a silken aspect, and this is 

 heightened by the shallow radiating grooves which cross it. Good illustrations 

 of such teeth from a large specimen are given in PI. XLVII, figs. 13 — 18. One 

 tooth of this series (fig. 13) is evidently from the lower middle row, while another 

 (fig. 14), shoAving especially well the passage of the median transverse ridges into 

 the marginal granulation, probably represents the inner paired row of the upper 

 jaw. The lateral teeth (figs. 15 — 18) are obliquely distorted, their middle portion 

 is comparatively small and low, and the marginal area is widest on the outer side. 



