228 



PALEONTOLOGY 



(Fig. 63, *). The ganoids were divided by Huxley into 

 Crossopterygii (fringe-finned), in which the paired fins have 

 a central skeletal axis with a fringe of fin-rays, and 

 Actinopterygii, in which the axis is wanting and the fin 

 supported by rays only. 



2. The Teleostei, or typical bony fishes, appear sud- 

 denly in the Upper Cretaceous. They are distinguished 

 by their thin, overlapping scales from the typical ganoids, 

 but some fishes reckoned as ganoids have similar scales. 

 When preserved as fossils the skeleton is usually fairly 



FIG. 65. CAUDAL FINS. 



a, Heterocercal fin of a shark, vertebral axis rising, second lobe of fin 

 developed below it ; b, homocercal fin of bony fish, vertebral axis as in 

 a, but the whole fin externally symmetrical ; c, fin of 'chthyosaurus, 

 externally like b, but vertebial axis running into lower lobe. 



complete, and the scales may also be preserved in 

 position. Isolated vertebrae are sometimes found and 

 are recognizable by their cylindrical bodies with very 

 deep conical hollows at each end : this biconcave or 

 " amphicrelous " type is, however, found also in the 

 lower grades of nearly all branches of air-breathing 

 Vertebrata. The neural arches which protect the spinal 

 cord may remain attached to or be detached from the 

 bodies ; in the tail there is a similar haemal arch below. 

 It is impossible to give further details within the neces- 

 sary limits of this chapter. 



