FOSSILS AND PEDIGREES 



355 



Sturgeons {Acipenseridae) . Unlike the other famiHes mentioned 

 above, some of which survived until the Cretaceous, the 

 Belonorhynchids died out during the Jurassic period. As the 

 Palaeoniscidae were approaching extinction during the Liassic 

 or Lower Jurassic epochs, their place was taken by another 

 family, the Chondrosteidae, arising from the main Palaeoniscid 

 stem and eventually leading on to the living Sturgeons, which 



Fig. 127. PRIMITIVE PALAEOPTERYGIANS. 



A. Restoration of Stegotrachelus finlayi (Palaeoniscidae), X J. (After Woodward 

 and White) ; b. Restoration of Cheirodus granulosus (Platysomidae), X about \. 



(After Traquair.) 



first made their appearance during the Tertiary era. Here 

 again the ganoid scales have undergone considerable degenera- 

 tion, and, apart from the presence of a series of branchiostegal 

 rays and one or two other minor characters, Chondrosteus and 

 its allies are not very unlike their existing descendants. 



The Bichirs [Polypterus) and their eel-like relative {Calamoich- 

 thys), which live in the rivers and swamps of tropical Africa, 

 and are the only existing members of the last Palaeopterygian 

 order, the CladisHa, were, until quite recently, regarded as 



