64 IOWA GEOLOGICAL SURVEY 



Associated with almost the earliest Protospondyli, there were 

 a few precocious fishes which evidently completed their verte- 

 bral column at once. This race, including such genera as Phol- 

 idophorus 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 dominant, 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 con- 

 sists only of two pieces on each side. The Isospondyli, as they 

 are termed, being thus provided with a completely bony internal 

 skeleton as well as completed fins, admitted of many more varia- 

 tions 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 eventually descend from their high estate 

 by degeneration. They exhibit all the peculiar changes in the 

 skull, upper jaw, and pelvic fins noticed as 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 early Eocene mammals. 



In reviewing the history of this chain of development that has 

 now been traced, we are struck with the fact that fundamental 

 advances in the grade of fish life have always been sudden and 

 have begun with excessive vigor at the end of a long period of 

 stagnation, while each advance has been marked by the fixed 

 and definite acquisition of some new character — an "expression 

 point", as Cope termed it, — which seems to have rendered pos- 

 sible, or at least has been an essential accompaniment of, a fresh 

 outburst of developmental energy. As we have seen, the suc- 

 cessive "expression points" among fishes were the acquisition 

 of (1) paddle-like paired fins; (2) shortened fin-bases but per- 





