During the Cretaceous Period 219 



as formidable and destructive for fish-life as were the ptero- 

 dactyls, and like the latter they became proficient sea- 

 hunters. 



Though the subject is more fully discussed in a later 

 chapter, reference might here be made to the evolutionary 

 derivation of the earliest teleostean fishes. As has been 

 generally accepted by leading ichthyologists of the past 

 quarter century or even longer, these were all derived from 

 holostean freshwater ancestry. In some cases, for example 

 as with the highly specialized pycnodont fishes — primitively 

 freshwater genera like Mesodon and Athrodon of the Lias, 

 Stonesfield Slate, Kimmeridge Clay and Purbeck strata, 

 passed seaward as bottom feeders, and without originating 

 more dominant forms, became purely marine dwellers, like 

 Anomoeodiis, Palaeobalistum and Pycnodus of marine Cre- 

 taceous and Eocene age. Unadapted in body, in fins, in 

 teeth, and in general build for successful defense, not to say 

 offense, they died out in Upper Eocene times. 



In contrast to the last, derivative forms of the Eugna- 

 thidae, Pachycormidae, Aspidorhynchidae, Semionotidae, 

 and Leptolepidae passed seaward during late Jurassic or 

 early Cretaceous times, and started the first teleostean in- 

 vasion there. Still other and even more abundant repre- 

 sentatives of the Semionotidae, Pholidophoridae, and Lep- 

 tolepidae varied by slow degree and multiplied abundantly 

 in lakes, as the progenitors of our existing freshwater teleo- 

 stean groups. 



As illustrating the trend of events during the Jurassic 

 and Cretaceous periods, the following sketch might here 

 be given of what seems to be a progressive and continuous 

 evolutionary line of fish-modification, from Triassic to late 

 Cretaceous age. The ganoid genus Semionotus became 

 widely spread over the world from the early to late Trias- 

 sic, and this wholly in freshwaters. By gradual reduction 

 of the fin-fulcra, of the bones in the mandible, of the basal 

 plates in the pectoral fins, of the intestinal spiral valve, 

 and by decussation of the optic nerves, transition is made 

 to the Pholidophoridae. 



The species of Pholidophorus were also freshwater, 

 and extended from the Keuper and Rhaetic or upper beds 



