DEVONIAN FISHES OF IOWA 63 



originally produced the earliest crawling lung-breathers or 

 Labyrinthodonts. No intermediate forms have hitherto been 

 discovered, while the links are still wanting between the simple 

 paddle of the fish and the five-toed or four-toed limb of the 

 Labyrinthodont ; but the Devonian and some later Crossopterygii 

 are the only fishes which agree with the Labyrinthodonts in 

 (1) the arrangement of their external head-bones; (2) the com- 

 plexity of their tooth-structure; (3) the possession of vomerine 

 tusks ; (4) the frequent presence of a pineal foramen in the skull; 

 and (5) the common occurrence of sclerotic plates around the 

 eye. These resemblances can scarcely be accidental, especially 

 considering the period at which they occur ; and it is one of the 

 problems of palaeontology to determine the exact relationships 

 between the paddle-finned fishes and the lung-breathers by the 

 discovery of perhaps Lower Devonian links. ' ' 



Passing now to the next higher grade of fishes, the Chon- 

 drostei, or Sturgeon tribe, which flourished especially during the 

 Carboniferous and Permian, we note this peculiarity of their fin- 

 structure : the internal cartilages formed only an effective basal 

 support for an expanse of membrane, which was stiffened by 

 flexible skin-fibres. The latter eventually formed fin-rays, and 

 articulated with the basal pieces when the cartilage' was re- 

 placed by bone. Thus arose the best form of appendage both 

 for balancing the body and for progression in water. The typical 

 Palaeozoic Chondrosteans having this type of fin were rapacious 

 fishes, and a few of them survived with little change until Upper 

 Jurassic times. Some of them degenerated into eel-shaped crea- 

 tures during the early Mesozoic, while others grew to unwieldy 

 proportions and eventually passed into the modern sturgeons. 

 The median fins became absolutely complete in the Proto- 

 spondyli, after the upper lobe of the tail had shortened so that 

 the caudal fin formed a flexible fan-shaped expansion at the blunt 

 end of the body, while each separate ray in the other median fins 

 was provided with its own definite support. The Protospondyli 

 characterized the Triassic and Jurassic periods, and exhibited 

 endless variety ; but their sole survivors at the present day are 

 the long-bodied Lepidosteus (garpike) and Amia (bowfin) of 

 American fresh waters. 



