LEPIDOID FISHES. 215 



remarkable than the Pycnodonts for their large rhomboidal 

 bony scales, of great thickness, and covered with beautiful 

 enamel. The Dapedium of the lias (PI. 1. Fig. 54.) affords 

 an example of these scales, well known to geologists. They 

 are usually furnished on their upper margin with a large 

 process or hook, placed like the hook or peg near the upper 

 margin of a tile ; this hook fits into a depression on the lower 

 margin of the scales placed next above it. (See PI. 27, 

 Figs. 3, 4, and PI. 15, Fig. 17.) All Ganoidian Fishes, of 

 every formation, prior to the Chalk, were enclosed in a 

 similar cuirass, composed of bony scales, covered with 

 enamel, and extending from the head to the rays of the 

 tail.* One or two species only, having this peculiar arma- 

 ture of enamelled bony scales, have yet been discovered in 

 the Cretaceous series ; and three or four species in the Ter- 

 tiary formations. Among living Fishes, scales of this kind 

 occur only in the two genera, Lepidosteus and Polypterus. 

 Not a single genus of all that are found in the Oohtic se- 

 ries exists at the present time. The most abundant Fishes 

 of the Wealden formation belong to genera that prevailed 

 through the Oolitic period. f 



♦ The Pycnodonts, as well as the fossil Sauroids, have enamelled scales, but 

 it is in the Leipidoids that scales of this kind are most highly developed. M. 

 Agassiz'has ascertained nearly 200 fossil species that had this kind of armour. 

 The use of such a universal covering of thick bony and enamelled scales 

 surrounding like a cuirass the entire bodies of so many species of Fishes, in 

 all formations anterior to the Cretaceous deposites, may have been to defend 

 their bodies against waters that were warmer, or subject to more suddeu 

 changes of temperature than could be endured by Fishes, whose skin was 

 protected only by such thin, and often disconnected coverings, as the mem- 

 branous and horny scales of most modern Fishes. 



-j- The most remarkable of these are the genus Lepidotus, Pholidophorus, 

 Pycnodus, and Hybodus. 



