THE GANOIDEI. 



127 



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

 taken into account, they form a large order, divisible into the 

 following sub-orders : 1 Aniiadce, 3. Lepidosteidae, 3. Grosso- 

 pterygidce, 4. C/iondrosteidce, all of which have living repre- 

 sentatives ; while the other three — viz., 5. Gephalaspidce, 6. 

 Placodermi, and 7. Acanthodidm — have been extinct since 

 the Palceozoic epoch, and are only ranged among the Ganoids 

 provisionally, inasmuch as we have no knowledge of their in- 

 ternal anatomy. 



1. The Amiadce have a single living representative in the 

 rivers of North America — Amia calva / and it is not certain 

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

 cycloid scales, preoperoulum, single median jugular plate, 

 branchiostegal rays, non-lobate paired fins, and heterocercal 

 tail, diagnose the sub-order. 



2. The Lepiidosteidce have rhomboidal enamelled scales, a 

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

 heterocercal tail. These are represented in the rivers of 

 North America at the present ^ay, and in tertiary formations, 

 by Lepidosteus ; in the Mesozoic rocks, by a great variety of 

 genera — Lepidotus, CEchmodus, Dapedius, etc. ; and, in the 

 Palaeozoic epoch, by Paleonisaus in the Carboniferous, and 

 probably by Gheirolepis, in the Devonian, formation. 



3. In the Grossopterygidoe the scales vary in thickness and 

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



Fia. 48.— Restoration of EoloptycMvA 



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

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

 the ventrals, are lobate ; they are sometimes rounded^ as in 

 Polypterus — sometimes greatly elongated and almost filiform, 

 as in IToloptychius (Fig. 43). There are no branchiostegal 

 rays, but twj principal, and sometimes many smaller lateral, 



