THE GANOIDEI. 



145 



When the fossil, as well as the existing Ganoiclei, are 

 taken into acconnt, they form a large order, divisible 

 into the following suborders : — 1. Amiaclce, 2. Lepidosteidce . 

 3. CrossopterygidcB, 4. Chondrosteidce, all of which have living 

 representatives; while the other three — viz., 5. Ceplmlaspidce, 

 6. Placodermi, and 7. Acanthodidce — have been extinct since 

 the Palaeozoic epoch, and are only ranged among the Ganoids 

 provisionally, inasmuch as we have no knowledge of their 

 internal anatomy. 



1. The Amiadce have a single living representative in the 

 rivers of North America — Amia ccdva ; and it is not certain 

 that any member of the group occurs in the fossil state. 

 The cycloid scales, preopierculum, single median jugular 

 plate, branchiostegal rays, non-lobate paired fins, and 

 heterocercal tail, diagnose the suborder. " 



2. The Lepklosteidae have rhomboidal enamelled scales, 

 a preoperculuni, branchiostegal rays, non-lobate paired 

 fins, and heterocercal tail. These are represented in the 

 rivers of North America at the present day, and in tertiary 

 formations, by Lepidosteus ; in the Mesozoic rocks, by a great 

 variety of genera — Lepidotus, (Echmodus, Dapediiis, &c. ; and. 

 in the Palaeozoic epoch, by Pcdeoniscus in the Carboniferous, 

 and probably by Cheirolepis, in the Devonian, formation. 



3. In the Crossopterygidce the scales vary in thickness and 

 ornamentation, and may be thin and cycloid, or thick and 



Fig. 43. — Restoration of Holojjtychius. 







rhomboid. The dorsal fins are either two in number, or, if 

 single, very long, or multifid. The pectoral fins, and usually 

 the veutrals, are lobate ; they are sometimes rounded, as 



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