36o A HISTORY OF FISHES 



two further, the body being even more eel-shaped, the paired 

 fins reduced to whip-Hke vestiges, and the small scales so much 

 embedded in the skin as to be invisible externally {cf. Figs. 

 99A, b). 



The sub-class Neopterygii had its origin in later Permian 

 times, and includes the vast majority of the Bony Fishes living 

 to-day. The main orders and sub-orders into which these 

 fishes are grouped will be discussed in the next chapter, and it 

 must suffice here to describe very briefly some of the earlier 

 families, and to indicate their relationship to the more general- 

 ised of the modern Bony Fishes. The earliest family is repre- 

 sented by the Semionotidae, which flourished during the Triassic 

 and Jurassic periods. They seem to have sprung from the 

 Palaeoniscid stem, and although there are a number of differ- 

 ences between the two types. Dr. Regan has suggested that 

 nearly all these can be interpreted as being related to a change 

 in the mode of life. The Palaeoniscids, it will be remembered, 

 were active, predaceous fishes, with a large mouth and strong 

 teeth, whereas the Semionotids were comparatively slow 

 swimmers, feeding at the bottom on shell-fish and the like, 

 and had a small mouth armed with teeth of a more speciaHsed 

 type. Just as the diflferences in the mouths and dentition can 

 be shown to be related to a change in the diet, so those concerned 

 with the form of the skull, pectoral arch, median fins, etc., 

 seem to be connected with a change in the swimming habits : at 

 the same time, however, other diflferences, among which the 

 structure of the scales may be mentioned, do not appear to be 

 so related. The family includes a number of diverse genera 

 from such widely separated localities as England, Europe, 

 South Africa, India, Australia, and North America. Some had 

 a typically fusiform shape, others were deep bodied: nearly 

 all were covered with ganoid scales (Fig. 131 a, b). 



During Triassic and Jurassic times the Semionotids gave rise 

 to a number of important oflfshoots, most of which flourished 

 for some time, but all became extinct before or during the 

 Cretaceous period, with the sole exception of the Bow-fin 

 (Amia) and Gar Pikes (Lepidosteus) , still Hving in North America. 

 The Pycnodontidae were remarkable fishes, bearing a superficial 

 resemblance to some of the modern File-fishes, with a deep 

 body, small mouth and specialised crushing teeth. They 

 ranged from the Jurassic to the Lower Eocene. The Eugnathididae 

 were large mouthed, predaceous forms, which made their first 

 appearance during the Triassic and flourished throughout the 



