404 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



elasmobranchs are found, however, as an abundant fauna of acanthodians 

 in the truly continental deposits of the Old Eed Sandstone, where not 

 only in grade of life, but in number of genera and species, they far sur- 

 pass the ostracoderms. In the clearly marine waters of the Lower De- 

 vonian, on the other hand, their record is scanty or doubtful. In the 

 Middle Old Eed Sandstone acanthodian sharks continue in the fresh 

 waters and compete with the erossopterygian ganoids. In the seas of 

 Middle Devonian time, probably later than the epoch of the Middle Old 

 Eed Sandstone, the more typical sharks become abundant and compete 

 with arthrodires. In the Upper Devonian the elasmobranchs disappear 

 from the British fresh-water record, but maintain their ascendency in 

 the sea. 



The sharks, though primitive in organization, are powerful, aggressive, 

 and not unintelligent fishes. Although experiencing times of repression 

 and expansion and passing into adaptations fitted for preying on new 

 sources of food, they have maintained their place as a subclass, of power- 

 ful fishes through all later ages. The geologic record shown here in detail 

 suggests, as previously argued, that their origin was in the fresh waters 

 rather than in those of the sea. The spines of the Silurian genus Onchns 

 are found in the deposits of more or less embayed or marginal waters and 

 are to be compared with the absence of a good geologic record of the life 

 of contemporaneous continental waters on the one hand, the absence on 

 the other hand of elasmobranch spines or teeth among the abundant re- 

 mains of invertebrates in the truly marine Silurian faunas. In the Lower 

 Devonian a better fresh-water record has been preserved. There the 

 acanthodian elasmobranchs are found to be the most important group of 

 fishes and are much more abundantly preserved than in any marine for- 

 mation below the Middle Devonian. Thus the actual record, although 

 not perhaps conclusive, is more suggestive of an origin and rise of elasmo- 

 branchs, the most primitive subclass of true fishes, in the flowing fresh 

 v-aters than in the sea. 



At present the sharks and rays are found invading harbors ; sharks pass 

 up into the mouths of large tropical livers, such as those of Indo-China; 

 and one species of shark is found in Lake Nicaragua, probably trapped 

 by the mountain growth which has made the lake basin. With this ex- 

 ception, however,- the group -is distinctly marine, though many species 

 exhibit a tolerance of brackish water. The present marine habitat stands 

 in marked contrast to the earliest view which we gain of them in the 

 Lower Devonian. If the elasmobranchs were, at the present time, still a 

 group showing free adaptations to both fresh and salt water, their disap- 

 pearance from the fossil fauna of the Upper Old Bed Sandstone could be 



