Summary of Conclusions Reached 515 



elasmobranchs took place. The studies of Young, Davis, 

 and Hind in Europe, also Newberry, Worthen and others 

 in America are considered. It is concluded therefore that 

 though some species remained wholly in freshwaters, others 

 became anadromous, while a considerable number became 

 marine. So though petroleum products had hitherto been 

 derived wholly from Silurian and Old Red beds, supplies 

 from S. Illinois at least seem to come from the Carbonifer- 

 ous Limestone series and from marine elasmobranchs. 



So the writer sums up conclusions regarding Carboni- 

 ferous fish life thus: "on a conservative basis it appears 

 that there were at least 55 genera and 360 species. Of 

 these about 20 genera and close on 100 species remained 

 as freshwater or anadromous species, the remainder be- 

 came largely or wholly marine." The freshwater ones 

 show westward extension also to Iowa, Missouri, and prob- 

 ably also to Nebraska and Kansas. 



But after deposition of the Coal Measures beds, some 

 widespread set of events blotted out the elasmobranchs 

 wholesale, for only 5 freshwater genera — Acanthodes, 

 Pleuracanthtis, Diplodus, Janassa, and fVodnika — survived, 

 though in greatly reduced number of species. The inland' 

 freshwater types then alone persisted till Jurassic time, 

 when a reinvasion of the sea took place. 



From papers by Roche, Fritsch, Cope, Hussakof and 

 others, the history of the surviving freshwater Permian 

 forms Is traced, and the extension of some of them even to 

 Australia is quoted from Woodward. The cestracionts 

 Aa-odus and Hyhodus of Upper Permian beds, seem to 

 have been the most abundant and persistent types of fresh- 

 water elasmobranch that lived on till Cretaceous time. 

 When reinvasion of the sea was made during the Jurassic 

 period, the Notldantidae, Scyllidae, Rhinidae, and Rhino- 

 batldae, all primitive groups of Rays, also Palaeospinax 

 and Cestracion, became the marine types, and have existed 

 there till present time. 



Chapter 10. The Dipneusti and Crossopterygii in time 

 and space. 

 Next to the Cyclostomata the former Is regarded as 

 probably the most primitive, continuous, and least variable 



