DEVONIC FISHES OF THE NEW YORK FORMATIONS 1 83 



precocious fishes which evidently completed their vertebral column at once. 

 This race, including such genera as Pholidophorus and Leptolepis, seems 

 to have temporarily exhausted itself in the effort, for it always occupied a 

 secondary place in the fish faunas until the beginning of the Cretaceous 

 period, when it rapidly multiplied, became fashionable, and replaced the 

 Protospondyli. Thus arose the modern fishes, of the same grade as the 

 herring and salmon, characterized not only by a complete vertebral column, 

 but also by a simplified lower jaw, which consists only of two pieces on each 

 side (without the splenial bone which forms so conspicuous a feature of the 

 earlier fishes). The Isospondyli, as they are termed, being thus provided 

 with a completely bony internal skeleton as well as completed fins, admitted 

 of many more variations than any of their forerunners. 



Among fishes, as among other animals, spines characterize only the 

 latest representatives of the class. The Acanthopterygii (" spine-finned ") 

 are thus the highest and latest fishes of all, though they sometimes eventu- 

 ally descend from their high estate by degeneration. They exhibit all the 

 peculiar changes in the skull, upper jaw, and pelvic fins noticed at first 

 appearing in a variable manner in the Cretaceous Isospondyli. . . The 

 spiny-finned fishes began by Berycoids and possibly Scombroids in the 

 Chalk, closely resembling, but not identical with genera living at the present 

 day. By the Eocene period, however, nearly all the modern groups of 

 Acanthopterygii had become completely separated and developed, and their 

 sudden appearance is as mysterious as that of the early Eocene Mammalia. 



GEOLOGICAL CONCLUSIONS 

 It will be convenient to include under this head certain topics whose 

 practical bearing is of chief interest to the geologist, although the evidence 

 involved is partly zoological, and in still larger part geographical, or paleo- 

 geographical. We refer to such matters as relate to the areal and vertical 

 distribution of Devonic fish life, the dispersion of new types and varieties, 

 migration, succession and occasional recurrence of faunas, and indications 

 furnished by the fossils themselves in regard to climatal and physical con- 

 ditions, either those of local nature, or others prevailing over wide areas. 

 Thus, by way of illustration, we are able to afifirm from the general com- 

 plexion of ancient faunas, that the climate of arctic regions was notably 

 warmer during the Devonic and late Paleozoic than at subsequent periods. 

 We are in possession, also, of a large fund of evidence regarding migra- 

 tional movements, and can delineate with great exactitude a number of 



