During Triassic and Jurassic Periods 175 



Now we would not only claim a freshwater origin for 

 these fishes, we would further suggest that the bituminous 

 material is a direct product of decomposition of oily con- 

 stituents set free from the decaying fish. The statements 

 quoted below from Newberry leave little doubt regarding 

 this, while the statistics already presented furnish proof 

 that wholesale destruction of a teeming freshwater fish- 

 life was the source for, and furnished the supplies of, the 

 Triassic bituminous material. 



In spite therefore of the apparent mixing of marine 

 invertebrate remains and of selachian bones or plates, or 

 the not infrequent intercalation of a bed of varying width 

 amongst typical marine dolomitic, chalk, or limestone de- 

 posits of great thickness, a careful study of the nature of 

 their deposits, of their included organisms, and of the bio- 

 logical relation of these to each other, reveals that Triassic 

 v^ertebrate life was still wholly freshwater, or that a very 

 few only of the predatory elasmobranchs and specially the 

 cestracionts were advancing seaward. 



When comparison is made of the fish-fauna of the above 

 four divisions, throughout Europe, it becomes evident that 

 the older selachian groups which lingered on into the Per- 

 mian have entirely disappeared, the Elasmobranchii even 

 as a great class seem to have temporarily suffered an eclipse 

 alike in numbers and in species. And when introduced to 

 us again later on in the Lias, they are found to be fresh- 

 water forms which are increasingly developing a marine 

 environment. The Dipneustei are no longer represented 

 by Dipterus, Uronemiis or Sagenodiis of the Palaeozoic 

 epoch, but have evolved the genus Ceratodus that closely 

 resembled the Barramunda of Australia. The Chondro- 

 steans are wonderfully rich in types that are intermediate 

 between palaeozoic and upper mesozoic genera. The older 

 types of crossopterygians have entirely disappeared, and 

 in their places are Graphiiirus, Diplurus and Undina, all 

 freshwater in habitat. 



Of the chondrostean "ganoids" that come Into marked 

 prominence, such genera as Dictyopyge, BeJonorhynchus, 

 Saurichthys, Semionotus, Colohodus, Heterolepidotus, Al- 

 lolepidottis, Pholidophonis, Thoracopterus, Pholidopleu- 



