The Primitive Fishes 



291 



Acrodus, Phoebodus, Ceratodus and Semionotus as well as 

 Estheria minuta. 



But increasing re-invasion of the sea by elasmobranchs 

 took place during Rhaetic and Lower Jurassic times, till 

 in Mid and late Jurassic marine rocks five families became 

 prominent. These were the Notidanidae, Scyllidae or dog- 

 fishes, the Rhinidae or angel sharks, the Rhinobatidae or 

 primitive rays, and two generic descendants of the cestraci- 

 onts viz, Palaeospinax and Cestracion or Heterodontus. 



A noteworthy feature of all of these groups is that 

 they persist, in representative genera, down to our time. 

 Furthermore all may be said to be marine. The question 

 then arises as to the centre or region, and also the time, 

 when the great elasmobranch phylum again became domi- 

 nant and reestablished itself in the sea? 



Pa/aeosjicnax 



Del&itijckcas , 



Cesfracf07?mae 



CocauoJo, 



AcroacLS 



Prime tfl/e P/^uT^aca/itAcc^ 



The writer believes that for the Cestracionts this can 

 be fairly readily established. For the very frequent occur- 

 rence of Hybodus and Acrodus in freshwater strata from 

 the Bunter and Muschelkalk, upward to the Wealden as 



