1 82 NEW YORK STATE MUSEUM 



could never be active swimmers, and before the close of the Paleozoic epoch 

 most of them had become extinct. The only widely spread family during 

 the Mesozoic epoch was that of the Coelacanthidae, which became degene- 

 rate through having lost some of its head bones and most of its tail ; while 

 the sole survivors at the present day are the eel-shaped Polypterus and 

 Calamoichthys found in the fresh waters of Africa. The Coelacanthidae 

 are often quoted as one of the best illustrations of a "persistent type," for 

 they scarcely changed between the Lower Carboniferous and the Upper 

 Chalk. They are, indeed, like most persistent types, organisms in which 

 structural changes took place in the wrong order — not in the order which 

 insured advancement. 



The Dipnoi, or paddle-finned fishes, which breathe both by gills and 

 by a modified air bladder (almost a lung), were also especially abundant 

 during the Paleozoic. In having the fundamental part of the upper jaw 

 fused with the skull instead of loosely suspended from it, the Dipnoi agree 

 more closely with the land animals than do the Crossopterygii ; but before 

 this feature had been acquired, the roof bones of the skull had subdivided 

 into smaller plates, such as could not have changed into the skull bones of 

 an ordinary Labyrinthodont, while the teeth had clustered into plates, so 

 that they could never have produced the Labyrinthodont dentition. The 

 essential change in the Dipnoan skeleton which led towards the land ani- 

 mals thus occurred too late, after other changes had rendered advance 

 impossible. The result was that with the dawn of the Triassic period the 

 Dipnoi made their nearest possible approach to the higher grade of life in 

 the genus Ceratodus, which still survives almost unchanged in the rivers 

 of Queensland ; while the latest members of the group degenerated to mere 

 eel-shaped animals (Lepidosiren and Protopterus), which live in the fresh 

 waters of South America and Africa. 



Development of effective fins. The next grade of fishes, the Chondrostei, 

 which specially characterized the Carboniferous and Permian periods, had 

 fins in which the internal cartilages formed only an effective basal support, 

 while the greater part of their expanse was stiffened by flexible skin fibers, 

 which had become " fin rays." Some of these fishes degenerated into eel- 

 shaped creatures in the Triassic, Rhaetic and Liassic periods, while others 

 grew to unwieldy proportions and eventually passed into the modern stur- 

 geons. . . The median fins became absolutely complete in the Proto- 

 spondyli, after the upper lobe of the tail had shortened so that the tail 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 and Amia of American fresh waters. 



Associated with almost the earliest Protospondyli, there were a few 



