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 Amiadce, 2. Lepidosteidce^ 'd. Grosso- 

 pterygidce, 4. Ghondrosteidce, all of which have li\nng repre- 

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

 Placodermi^ and 7. A.cantJiodldoe — have been extinct since 

 the Palaeozoic 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 — Amla calva ^ and it is not certain 

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

 cycloid scales, preoperculum, single median jugular plate, 

 branchiostegal rays, non-lobate paired fins, and heterocercal 

 tail, diagnose the sub-order. 



2. The Lepidosteidce have rhomboidal enamelled scales, a 

 preoperculum, 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 Lejndosteus j in the Mesozoic rocks, by a great variety of 

 genera — Lepidotus^ (Echrnodus^ Dapedlus^ etc. ; and, in the 

 Palseozoic epoch, by Paleonisciis in the Carboniferous, and 

 probably by Ghelrohpis^ in the Devonian, formation. 



3. In the Grossopterygid(B the scales vary in thickness and 

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



Fig. 43. — Eestoration of HoloxAycMus. 



or, if 



rhomboiii The dorsal fins are either two in number, 

 single, very 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 irolop*,ychius (Fig. 43). There are no branchiostegal 

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



