§ 21. FISHES OF THE PERMIAN. 565 



sixteen or seventeen genera ; and with, but one exception, 

 (the Palceonhcus Freislebeni of the upper eoal-measures), 

 are peculiar. All the- ichthyolites of this group possess 

 that remarkable modification of the tail, which we have 

 already mentioned as having been of excessive rarity in the 

 fishes of the secondary and tertiary seas, as well as in those 

 that inhabit the existing ocean ; in which the Sharks and 

 Sturgeons are almost the only representatives of this 

 palaeozoic type. The caudal fin is universally heterocercal, 

 i. e. the vertebral column is prolonged at its extremity into 

 the upper lobe of the tail.* 



Ganoid fishes of the genera Pal&oniscus.f and Platy- 

 somus ; Cestracions,^ Hybodonts ; and a remarkable form 

 of those extraordinary fishes, the Chimeroids, named 

 Ceratodus ;§ constitute the principal features of the Per- 

 mian ichthyic fauna. Species of the two first genera have 

 been found in abundance in various localities in England, 

 on the Continent, and in North America, and are figured 

 and described by various authors. || Their remains are the 

 only relics of vertebrated animals hitherto discovered in 

 the sandstones impressed with the footsteps previously 

 examined.^" 



* See Lignograph of the restored figure of the Amblypterus of the 

 Carboniferous system ; in which a marks the prolongation of the ver- 

 tebral column into the upper lobe of the caudal fin : this fish is a good 

 illustration of the heterocercal type; and the Lepidotus of the Wealden, 

 (Lign. 99, p. 408,) affords an example of the homocercal, or equal-lobed 

 tail. 



f Medals, vol. ii. p. 633. 



I Medals, vol. ii. p. 613. 



§ Medals, vol. ii. p. 618. The teeth or dental plates of several 

 species of Ceratodus, occur in the New Red of Germany, especially at 

 Hohneck near Ludwigsburg. 



|| Professor Sedgwick's memoir is accompanied by numerous 

 figures of Palseonisei. — Geol. Trans, vol. iii. pi. 12. 



! The characters of the Ichthyology of the New Red are thus 

 expressed by M. Agassiz, ' Les formations triassiques et le Zechstein 

 pp 2 



