ORDERS OF FISHES. 133 



structure characterises a division of Ganoids called by Hux- 

 ley, for this reason, Crossopterygidm, or " fringe-finned." The 

 form of the caudal fin varies, the Ganoids being in this re- 

 sjDect intermediate between the Bony fishes, in which the 

 tail is " homocercal," and the Sharks and Eays, in which 

 there is a " heterocercal " caudal fin. In the majority of 

 Ganoids, then, the tail is unsymmetrical or " heterocercal," 

 but it is sometimes equi-lobed or "homocercal." 



The living genera of Ganoids are exclusively or mainly 

 inhabitants of fresh waters ; but many of the extinct forms 

 occur in association with marine animals, and must therefore 

 be assumed to have inhabited the sea. Others, again, are 

 found in undoubted lacustrine deposits ; and lastly, there are 

 others which are found in beds where there are no other 

 fossils which can be certainly asserted to have lived either 

 in fresh or salt water ; and as to the mode of life of these 

 we must at present remain in doubt. 



As regards their general distribution in time, the oldest 

 representatives of the fishes belong, so far as is yet known 

 with certainty, to this order. The order, namely, is repre- 

 sented in the Upper Silurian rocks of Bohemia and Britain 

 by several Ganoid fishes, which have been referred to five 

 distinct genera. In the Devonian rocks, or Old Eed Sand- 

 stone, the Ganoids attain their maximum. The singular 

 family of the Ce^phalaspidce appears to die out finally at 

 the close of this period, and the great group of the Crosso- 

 23terygidcB attained here its highest development, being repre- 

 sented at the present day by the single genus Polypterus. 

 The Carboniferous and Permian rocks contain an abundance 

 of Lepidoganoids. In the Mesozoic period, the Lepidoganoids 

 are very largely represented by various extinct types, many 

 of which belong to the family of the Leindosteidm — repre- 

 sented at the present day by the Bony Pike or Gar-pike of 

 North America. Here, also, we have for the first time rep- 

 resentatives of the family of the Chondrosteidce, to which the 

 living Sturgeons belong. Lastly, iu the Oolitic rocks appear 

 for the first time Lepidoganoids with homocercal tails, and 

 they continue to be represented up to the present day. In 

 the Tertiary rocks true Sturgeons {Acipenser) make their 



