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THE CHRONOLOGICAL DISTRIBUTION 



apparently almost complete extinction of the Elasmobranchs during the Permian period. 

 Only about ton species have, so far, been reported from North America, and it is possi- 

 ble that there were even fewer, since tbe reference of certain deposits in Eastern Illinois 

 to the Permian is perhaps questionable. In Europe about twenty species have been 

 described, the majority of them by Fritsch, within recent years. 



Diagram illustrating distribution of fossil Elasmobranchs, 



It is further seen from the diagram that at once after tbe beginning of the Triassic 

 period the number of species of Elasmobranchs rose rapidly in Europe, while in North 

 America the number apparently even diminished; since only about three species are 

 known to have existed during the time of the laying down of the Jura-Trias. Doubtless 

 this apparent dearth of species is, partly at least, due to tbe scant development of Trias- 

 sic and Jurassic rocks in our country. 



It might be supposed that in both North America and Europe the conditions dur- 

 ing the Permian had been unfavorable for the existence of the genera and species of Elas- 

 mobranchs which had flourished in such numbers during the Bubcarboniferous ; that 

 accordingly they had been driven to some other quarter of the globe ; and that it 

 would be found that in some of the Mcsozoic periods they had returned to the shores 

 occupied by their Palfflozoic ancestors. On this point two things are to be noted : First, 

 we get little or no light on the problem from Permian and early Triassic deposits in Other 



