Vol. 60.] JAWS OF PTYCIIODIS FKOM THE CHALK. 135 



this series in P. decurrens, but as they are traced forward towards 

 the eud of the S3'mphysis, they become shorter in proportion to their 

 width and more obliquely distorted, while their median coronal 

 elevation is smaller (fig. 5). The remaining lateral teeth are more 

 or less displaced, but they are clearly arranged in six paired series 

 altogether (fig. 2) ; and near the back of the symphysis the teeth 

 have the following transverse measurements (in millimetres) : — 

 o', 4 ; i', 6*5 : n', 5 ; in', 4 ; iv', 3*5 ; v', 3 ; vi' (?). 



The specimen therefore proves that, notwithstanding the powerful 

 nature of the grinding-dentition of Ptychodus and the straightness 

 of its transverse rows, the supporting jaws had not assumed the 

 peculiarly-effective disposition characteristic of the living Mylio- 

 batidae, as I formerly supposed. Although it is quite likely that the 

 angle between the mandibular rami seen in fig. 2 (PI. XV) is slightly 

 altered by accidental distortion, the fossil clearly shows that this 

 cannot have exceeded a right angle. The teeth must thus have 

 been supported by the elongation of the symphysis, of which there 

 is very distinct evidence. This arrangement is peculiar, not to the 

 MyUobatidae, but to the closely-allied Trygonidae, which have often 

 a powerful dentition. In fact, while Ptychodus is most closely 

 related to the Alyliobatidse by its teeth, as pointed out on former 

 occasions, 1 it is now shown to resemble the Trygonidae by its jaws. 



The probable explanation of the new discovery is that, in the 

 Cretaceous Period, the great rays of the 'families' Myliobatidae and 

 Trygonidae had not become fully differentiated. Prof. 0. Jaekel ' has 

 already arrived at such a conclusion from general considerations, 

 and proposed to place all these fishes in one comprehensive family 

 termed Centrobatidae. If this arrangement be adopted, Pty- 

 ihodus represents a primitive sub-family, Pty chodontinse, which 

 still awaits definition from lack of complete specimens ; while the 

 Trygoninae, Myliobatinae, and Ceratopterinac are equivalent sub- 

 families which survive at the present day. 



EXPLANATION OF PLATE XV. 



Ptychodus decurrens, Ag. ; remains of jaws and dentition, natural size, with 

 two teeth (figs. 4 & 5) enlarged twice. — Lower Chalk (zone of Holaster 

 subglobosus) ; Glynde, near Lewes, Sussex. Collection of Henry Willett, 

 Esq. nid = mandible ; ptq = upper jaw : o-V = teeth of lower jaw ; 

 o'-vi' = teeth of upper jaw. 



Fig. 1. Lower aspect, without symphysis. 



2. Upper aspect, showing the extent of the decayed symphysis. 



3. Part of upper dentition, oral aspect; 3 a, posterior end-view. 



4. Posterior tooth of the upper first paired series, left side. 



5. Anterior tooth of the same series. 



1 A. S. Woodward, Quart. Journ. Geol. Soc. vol. xliii (1887) p. 129; also 

 Proc. Geol. Assoc, vol. x (1888) pp. 294-98, and ' Catal. Foss. Fishes Brit. Mus.' 

 pt. i (1889) pp. 132-52. 



2 • Die eocanen Selachier toiu Monte Bolca' 1894, pp. 115-38. 



