PTYCHODUS. 



229 



pedicle of the neural and haemal arches was attached, the concentric laminae of 

 the centrum are penetrated by a deep cavity ('/.), which is filled by matrix in the 

 fossils, but must have been occupied originally by uncalcified cartilage. 



It is interesting- to observe that nearly similar vertebral centra occur in the 

 Lamnid shark, Gorax {supra, p. 1 ( .*~), and one specimen has been referred to 

 SelacheJ which is the existing genus most closely related to Gorax. It is, there- 

 fore, now uncertain whether the Selachian vertebras from the Cretaceous of 

 Antarctica which have been referred to Ptychodus, 2 really pertain to this genus. 



Ptychodus is widely distributed in Upper Cretaceous formations in Europe 

 and North America, but it appears and disappears suddenly in both regions. It 

 begins with the low-crowned and simply ridged teeth of P. decurrens, which at 

 first seem to represent a comparatively small variety ; and it ends with teeth 

 which are either very high-crowned {P. rvgosus) or marked with gyrate ridges 

 (P. polygyrus). Its predecessors have not hitherto been recognised, but the teeth 



Fig. 73. Apocopodon sericeus, Cope ; teeth of type specimen, nat. size.— Upper Cretaceous ; Maria Farinha, 

 State of PeiTiamhuco, Brazil. A. Median tooth, coronal view, nat. size, with (B) part of its superficial 

 gano-dentine, much magnified ; C. side view of the same tooth ; D. teeth of first and second inner paired 

 lows, anterior view; E. same teeth, lower or attached surface. 



of Ptychodus seem to be connected with those of the Tertiary Myliobatidas 

 by the dentition named Ajpocojpodon, which occurs at the top of the Cretaceous 

 series in South America. 3 In this genus (Text-fig. 73) the teeth are more or less 

 distinctly quadrangular (a), with ends bevelled as irregularly as those of Ptychodus. 

 The coronal surface is flattened and invested with a thin layer of gano-dentine, 

 which is marked with fine antero-posteriorly-directed wrinkles (u). The root is 

 sharply constricted from the crown (c), and differs from that of the teeth of 

 Ptychodus in being very slightly marked on its lower face with a few shallow, 

 broad, antero-posterior grooves (n, e), which become deeper and more numerous in 



1 Selache davisi, C. Hasse, " Eiuige seltene palaontologische Funde," Palseontogr., vol. xxxi (1884), 

 p. 9, pi. ii, figs. 16, 17. 



2 A. S. Woodward, " On Fossil Fish-remains from Snow Hill and Seymour Islands," Wissensch. 

 Ergeb. Suhwed. Sudpolar-Exped., 1901-03 (1908), vol. iii, pt. 4, with plate. 



3 E. D. Cope, " A Contribution to the Vertebrate Palaeontology of Brazil," Proc. Amer. Phil. Soc, 

 vol. xxiii (1886), p. 2. — A. S. Woodward, " Notes on Some Upper Cretaceous Fish-remains from 

 Sergipe and Pernambuco, Brazil," Geol. Mag. [5], vol. iv (1907), p. 194, pi. vii, figs. 4, 5. 



