During the Cretaceous Period 215 



began rapidly to dwindle toward extinction in the Upper 

 Eocene. 



Alongside both of these, moreover, various derivative 

 offshoots of freshwater teleosts, which — as traced in 

 chapter VI — had themselves originated from more ancient 

 and primitive "ganoids," were now adapted to a marine 

 environment. These included some large powerful and 

 voracious fishes like Poi-theus (Fig. 29, p. 227), Ichthyo- 

 dectes, and Enchodus, as well as others listed in later 

 tables (p. 226). 



The abundant primitive freshwater elasmobranchs of 

 late Old Red or Carboniferous age, like Acanthodes and 

 Pleuracanthus, were all elongate fusiform in shape, and 

 were much better provided with defensive spines, as well 

 as offensive teeth, than their congeners of other groups. 

 But after reaching a high specialization as fusiform types, 

 or after evolving into flattened types like Ctenacanthus and 

 Janassa respectively, amid marine environment, these, as 

 well as great groups of their freshwater ancestors were 

 obliterated wholesale during the Permian period and only 

 slowly recovered in freshwater, still more slowly sent out 

 derivatives into the sea in Jurassic time. The last again 

 evolved into two similar main lines as before — the fusiform 

 series or Squalidae, and the flattened series or Raiadae, 

 The former of these two retained on the whole the fusiform 

 body, the lithe movements, and the predaceous habits of 

 their preexisting freshwater ancestors; the latter became — 

 through intermediate types like Rhina or Squat'tna (Fig. 

 26, p. 194) and Rhinobatiis of probable Jurassic origin, 

 modified into flattened ground-feeders of highly specialized 

 body-structure, like Raja — the skates — and Cyclobatis both 

 first known to us from Upper Cretaceous rocks. 



In England the earlier works of Agassiz, Mantell, 

 Dixon and E. T. Newton on the Sussex and related beds, 

 but specially the elaborate work of A. S. Woodward on 

 "Fossil Fishes of the English Chalk" {157) illustrate how 

 richly the sea had become populated with elasmobranch 

 and chimaeroid species, also that chondrosteans allied to 

 the sturgeon as well as teleosteans of several families, were 

 abundant over a sea that covered the south-east part of the 



