322 ORDERS OF FISHES. 



As regards 'the general distribution in time of the Ganoidei, 

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

 known with certainty, to this order. The order, namely, is 

 represented 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 Red 

 Sandstone, the Ganoids attain their maximum. The singular 

 family of the Cephalaspida appears to die out finally at the 

 close of this period, and the great group of the Crossopterygida 

 attained here its highest development, being represented 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 Lepidosteida represented at the present day 

 by the Bony Pike or Gar-pike of North America. Here, also, 

 we have for the first time representatives of the family of the 

 Sturionidce, to which the living Sturgeons belong. Lastly, in 

 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 (Aci- 

 penser) make their appearance ; but the Ganoids are now con- 

 siderably outnumbered by the Teleostean fishes ; and the latter 

 have a still more marked predominance at the present day. 



The classification of the Ganoid Fishes has hitherto proved a 

 matter of extreme difficulty ; and probably no arrangement 

 that has been as yet proposed can be regarded as being, in all 

 its details, more than provisional. A convenient primary 

 division is that into Lepidoganoids, in which the body is 

 furnished with scales of moderate size, and the endoskeleton 

 is generally more or less perfectly ossified ; and Placoganoids, 

 in which the skeleton is imperfectly ossified, and the head and 

 more or less of the body are protected by large ganoid plates, 

 which in many cases are united together by sutures. Accept- 

 ing this division, the order Ganoidei may be divided into the 

 following sub-orders : 



SECTION i. LEPIDOGANOIDEI. 

 Sub-order A. Amiada. 



B. Lepidosteidce. 



C. Lepidopleuridce. 



D. Crossopterygidte. 



E. Acanthodidce. 



SECTION 2. PLACOGANOIDEI. 

 Sub-order F. Ostracostei. 

 G. Sturionida. 



