THE FOSSIL FISHES OF THE CHALK.
flattened of existing skates (such as Myliobatis). It is now
known, however, that the mouth was not a straight transverse
slit, but bore the teeth on a stout and elongated front portion
(symphysis) of the jaw (fig. 1). Ptychodus was therefore
doubtless less depressed in shape, more like some of the
existing skates of the family Rhinobatidae. The origin of the
genus is obscure, for although the oldest teeth (from the Chalk
Fig. 1. Mandible of Ptychodus decurrens, Agassiz.
Marl) are the smallest and least strongly ridged, it is difficult
to recognise their relationship to any teeth found in the
marine Cretaceous rocks immediately beneath. It can only
be said that still smaller crushing teeth, more transversely
elongated, which must have been arranged in the jaw of a
skate in the same way, occur in the estuarine Wealden
formation (Hylaeobatis).2 They may perhaps have belonged
2 A. S. Woodward, Fossil Fishes of the English Wealden and Purbeck Formations, Mon. Palaeont.
Society, 1916, p. 19.