72 



THE CHRONOLOGICAL OISTKI r.UTION 



being made was not favorable for their existence ; that consequently they migrated else- 

 where ; and that when they returned, so much time had elapsed that they had necessarily 

 become transformed beyond recognition. We must, however, keep in mind that the 

 Elasmobranchs of the Subcarboniferous period were evidently mostly dwellers in clear 

 open seas, that such seas were probably always accessible to such free-swimming animals, 

 and thai the space of time in question lias been spanned by more than one family of the 

 higher fishes. The Ccelacanthidse are known to have survived from the Devonian to 

 the Cretaceous. The Palseoniscidse likewise began in the Devonian and did not perish 

 until in middle Mesozoic times. During the Subcarboniferous period there existed over 

 forty species. With, each succeeding period the number of species became reduced, hut 

 the vis viva of the family was such that it penetrated into the .Jurassic. In the Permian 

 period there existed about thirty known species; in the Trias, fifteen species. None of 

 the genera, however, existed in both Palaeozoic and post-palfleozoic times. 



The Semionotidse furnish another example of a family which lived through the 

 transition from Palaeozoic to later times. It began in the Permian in a feeble way, with 

 a single genus and some live or six species; but the stock possessed such vigor that it 

 was able to endure until in the Cretaceous. In Triassic times there existed a, dozen 

 genera and nearly seventy species. The Permian genus Acentrophorus is believed to he 

 represented in (Ik; Triassic by a, single American species; hut the identification is in 

 doubt. 



It seems probable, therefore, that the failure of the Palaeozoic families of Elasmo- 

 branchs to perpetuate themselves beyond the Permian period was due more to the organi- 

 zation of the animals themselves than to any specially unfavorable environment. Their 

 vitality seems to have become reduced, and for that reason they perished. This is also 

 indicated by the fact that after the; Subcarboniferous period very few new genera were 

 evolved. Among the Pahconiseida 1 , on the contrary, new genera were brought into 

 existence with each succeeding period up to near the end of their history. The same 

 is true of the Semionotidse, although their course was a briefer one. Among the Elas- 

 mobranchs themselves, we find that when, after the middle of the Trias, they entered on 

 a, new career, the production of new genera, went on for a time with energy. Only a 

 few appeared in the; Trias, many in the Jurassic, a, greater number in the Cretaceous, many 

 in the Eocene. Since then that pristine vigor shown in the organization of new "-enera 

 has apparently declined ; and we may consequently regard the Elasmobranchs of to-day as 

 a moribund race. The seas of the tropical regions are now the homes of the greater 

 number of living forms, and the Tertiary rocks of the tropics have not been thoroughly 

 enough explored to enable us to say how the number of the Miocene species compares with 

 that of the living species ; but the Miocene sharks and rays of Europe far outnumber 



