326 Evolution and Distribution of Fishes 



the writer has failed to learn exactly as to their environal 

 habitat. Until the exact rock stratum that furnishes these 

 has been studied and associated organisms — if any — are 

 known, it would be idle to speculate regarding them. 



The scant remains of Acipenser, suggest that passage 

 seaward was a relatively recent habit, for Leidy's plate 

 from the Miocene, that of E. T, Newton from the fresh- 

 water Forest Beds, and the fin-spine from southern France, 

 seem all to indicate freshwater, rather than an anadromous 

 habit. 



The additional two genera now alive, Scaphirhynchus 

 and Kessleria (if one separates the three eastern from the 

 single western one generically) , are found, the former in 

 the Mississippi valley and Western States, the latter in the 

 rivers of Tartary. 



The family Polyodontidae that includes the Spoon-bill 

 sturgeons, are modified derivatives probably of some an- 

 cient acipenseroid type, in which the rostrum has become 

 greatly elongated and flattened. The scales of the trunk 

 are small obliquely-placed discs in some fossil forms {Cros- 

 sopliolis), or almost absorbed in the living Polyodon; the 

 larger rhombic scales are restricted to the tail region; the 

 mouth is wide and gaping not tubular as in the sturgeons; 

 small persistent teeth line both jaws as compared with the 

 edentulous state of adult sturgeons. 



The oldest known type of the family is Crossopholis 

 that was described by Cope from the freshwater Eocene 

 of Wyoming, but if Pholidurus is to be assigned here it is 

 reported from the Senonian and therefore older Cretaceous 

 beds of Kent. 



The Holostei. This great order that for some authors 

 includes two divisions the Protospondyli and Aetheos- 

 pondyli is, like most of the preceding groups, known largely 

 in the fossil state. But unlike these the order may be said 

 to be Mesozoic in history, if we except a few species of the 

 genus Acentrophorus of Permian age. The origin of the 

 Holostei, however, must therefore be looked for in that 

 age. Woodward's statements { i6q: Introd. VIII) ar'e 

 at once guarded, and represent the opinion of the highest 

 authority. He says: "The Catopteridae of the Trias incline 



