148 THE ANATOMY OF VEETEBRATED ANIMALS. 



The Pycnodontidce, wliicli are commonly grouped among 

 tlie Ganoids, are fishes with much-compressed bodies, like the ; 

 John Dory or the Filefishes, covered with large rhomboidal 

 enamelled scales, from which bony ridges projected inter- ■ 

 nally, and were imbedded in the integument. The noto- 

 chord is persistent, but the neural arches and the ribs 

 are ossified. The proximal ends of the ribs, imbedded in the 

 sheath of the notochord, are but little expanded in the more 

 ancient members of the groiip, while, in the more modern 

 species, they enlarge, and at length unite by serrated su- 

 tures, giving rise to spurious vertebrae. The skull is high 

 and narrow, as in Balistes ; the premaxillse are small, and 

 there are no teeth in the maxillae, but several longitudinal 

 series of crushing teeth (the vomer and parasphenoid ? ) 

 are attached to the base of the skull. These bite between 

 the rami of the mandible, which are also armed with several 

 rows of similar teeth. The teeth of the Pycnodonts have no 

 vei'tical successors. The pectoral fins are small, the ventral, 

 obsolete. The Pycnodonts are all extinct, but existed. 

 formerly, for a very long period of time — their fossil remains 

 occurring in rocks from the Carboniferous to the older 

 Tertiary formations, inclusively. They present curious 

 features of resemblance to the plectognath Teleosfei. 



The remains of Ganoid fishes began to appear in the 

 Upper Silurian rocks at the same time as those of the Ulas- 

 mohrancJiii, with which they constitute the oldest Yerte- 

 brate Fauna ; they abound in the Devonian formation, and 

 constitute, with the Elasmobranchii, the whole of the Palaeo- 

 zoic Fish Fauna. We are in ignorance of the true affinities 

 of Tharsis and Thrissops, and of the Hoplopleuridce ; but 

 unless some, oi- all, of these are Teleosteans, Ganoids and 

 Elasmobranchs, alone, constitute the Fish Fauna of the 

 Mesozoic formations, as far as the bottom of the Cre- 

 taceous series. 



Y. The Teleostei. — The osseous fishes are occasionally 

 devoid of any exoskeleton. Sometimes they present scat- 

 tered dermal plates of true bone ; or, as in the Triinkfishes 



