Chimceroidei, etc. 



15 



the earliest fossils referred to this order, but there are no examples 

 in the collection. Numerous teeth of Ganodus occur in the Great WaH-case, 

 Oolite of Stonesfield, near Oxford; Aletop acanthus is Liassic ; Table'cases 

 Ischyodus is known from the Jurassic, Gault, and Greensand ; and n 0Si 33 34] 



Fig. 30.— Lower Jaw of Edaphodon leptognathus, Middle Eocene, Bracklesham Bay, • 



Sussex. 



Edaphodon (Fig. 30) is Cretaceous and Eocene, while Elasmodus 

 occurs only in the latter formation. 



There can be no doubt that the spines known under the name of 

 Leptacantkus pertain to the dorsal fins of these Chimgeroids, being 

 found in association with their teeth from the Lias to the Chalk. 



PISCES IXCEETJE SEDIS. 



Group I. — Acaxthodini.* 



The Acanthodians form an interesting group of very small or Wall-case, 

 moderate- sized Palaeozoic fishes characterized by the presence of No - 4. 

 strong defensive spines in front of the paired and median fins, Table-cases, 

 but not in front of the caudal fin. They have minute rhombic Nos - 34 > 35 - 



Fig. 31. — Dip!aca?ithus st?-iatus, Old Red, Cromarty. 



d 1, first dorsal fin ; d 2, second dorsal fin ; p, pectoral fin ; v, pelvic fin ; a, anal fin ; 



c, caudal fin. Jtach fin has a strong defensive spine in front. 



* We have felt some doubt as to the propriety of placing the Acanthodini, 

 the Placodermata, and the Crphtilo.spid'P, under the order Ganoidei: from the 

 observations of Prof. Huxley, Dr. R. H. Traqudr. and others, it seems quite 

 probable that these ancient types of fishes, when bttter understood, may hereafter 

 form more than one distinct order. 



