288 Evolution and Distribution of Fishes 



in the Upper Trias ( Keuper) of England." Tlie "finely 

 preserved state shown by it and all of the others, indicates 

 a sudden death and entombment in a volcanic dust-shower 

 that gave rise to the 'dark indurated shale' in which all 

 were found." 



In continuing to trace the elasmobranchs upward through 

 the geologic scale, an almost complete break occurs from 

 the Mid Permian to the top of the Triassic formation. 

 Few genera as such survived from the Permian into the 

 Triassic, so far as we at present know. In the above 

 quotation from Woodward it is suggested that possibly 

 Pleiiracanthiis lingered on into Triassic time. The group 

 however that persisted most directly was that of the Cestra- 

 ciontidae or Port Jackson sharks. Not only so, if the view 

 entertained by some be correct the abundant genera Orodiis 

 and Campodus of Carboniferous and Coal Measure strata 

 — where they are known mainly by teeth — were continued 

 upward into Mesozoic life, as the equally abundant genus 

 Hybodtis (Figs. 24, 25, p. 193). For in all three genera 

 the teeth are practically alike, and the fin-spines closely 

 agree. But these unfortunately are the only parts pre- 

 served. The above two Carboniferous genera are rarely 

 freshwater in a few species, mainly they are marine. Hyho- 

 diis probably was descended from one of the former, for 

 it remained in freshwater. 



Nearly allied, and often found in the same strata as 

 Hybodtis, are teeth and spines of Acrodus. Both show 

 a wide distribution in freshwater across Central Europe 

 up to the close of the Wealden period, though an entire 

 absence from America, till Cretaceous times. But both 

 occur in freshwater beds of the Bunter, Muschelkalk, 

 Lettenkohle — where they are associated with the fresh- 

 water crustaceans Estheria minuta, with Ceratodus and 

 with the labyrinthodont, Mastodonsaiirus — the Keuper, 

 Rhaetic, Lower Lias, Stonesfield Slate, Lower Kimmerid- 

 gean, Purbeck and Wealden. But while Hybodtis seems 

 then to have become extinct, Acrodtis seems meanwhile to 

 have migrated, with other and recently evolved allies, into 

 marine surroundings, and then to have expanded its range, 

 during late Jurassic and specially Cretaceous times, into 



