PISCES 



135 



Order IV. Teleostei 



The teleosts as a group appeared first in Jurassic times, evidently 

 as a derivative from the holostean ganoids. Some of the more primi- 

 tive members of the teleostean sub-order Malacopterygii, have cer- 

 tain structures that are reminiscent of the Holostei, e. g., ganoid 

 scales, multivalvular conus arteriosus, fulcra in connection with the 

 fin bases, and spiral-valve in the intestine. In the higher teleosts 



Fig. 73. — Showing the embryos of the fish Rhodeus amarus Uving parasitically 

 in the gill cavities of the clam, Uriio. e, embryos; g, interlamellar cavities; i. l.j, 

 an inter-lamellar junction. (From Bridge, after Olt.) 



none of these characters are found. The teleosts illustrate better 

 than almost any other vertebrate group the principles of adaptive 

 radiation (Fig. 51). They started modestly in the Jurassic, increased 

 rapidly through both Lower and Upper Cretaceous, while in the Ter- 

 tiary they had radiated adaptively into all of the principle types of 

 body form that characterize the modern condition. Wlien we say 

 that the teleosts are a modern group we do not forget that the group 

 originated many millions of years ago and has been a dominant group 



