remarkable than the Pycnodonts for their large rhomboidal bony scales, of great thickness, and covered with beautiful enamel. The Dapedium of the lias (Pl. 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 Pl. 27, Figs. 3, 4, and Pl.,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.[1] One or two species only, having this peculiar armature of enamelled bony scales, have yet been discovered in the Cretaceous series; and three or four species in the Tertiary 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 Oolitic series exists at the present time. The most abundant Fishes of the Wealden formation belong to genera that prevailed through the Oolitic period.[2]
- ↑ The Pycnodonts, as well as the fossil Sauroids, have enamelled scales, but it is in the Lepidoids that scales of this kind are most highly developed. M. Agassiz has ascertained nearly 200 fossil species that had this kind off 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 sudden changes of temperature than could be endured by Fishes, whose skin was protected only by such thin, and often disconnected coverings, as the membranous and horny scales of most modern Fishes.
- ↑ The most remarkable of these are the genus Lepidotus, Pholidophorus, Pycnodus, and Hybodus.
