FISHES 



thys (Carboniferous and Lower Permian of Europe and North 

 America). 



Fam. 2. Rhizodontidae. Scales cycloid and overlapping. 

 Paired fins obtusely lobate. Tail heterocercal, sometimes appar- 

 ently gephyrocercal. Teeth with the external enamelled layer 

 of dentine infolded towards the axis in the form of radially 



Fia. 274. Skull of a Rhizodont (Rhizodopsis sauroides), Lower Carboniferous. A, 

 lateral view ; B, the dorsal surface ; and C, ventral view, an, Angular ; d, 

 dentary ; /, frontal ; i.d, infra-dentary ; j, principal jugular plates ; l.j, lateral 

 jugulars ; m, mandible ; m.j, median jugular ; mx, maxilla ; o, orbit ; op, oper- 

 culum ; p, parietal ; p.f, post-frontal ; p.mx, premaxilla ; p. op, preoperculum ; so, 

 suborbital ; s.op, suboperculum ; sq, squamosal ; st, supra-temporal ; x, x, cheek 

 plates. (After Traquair.) 



arranged folds. In some genera ring -like vertebral centra 

 have been recognised and also a preoperculum. Genera : 

 Rhizodus, Lower Carboniferous of Scotland and Northumberland ; 

 Tristichopterus 1 (Fig. 275), Old Eed Sandstone of Scotland; 

 JZusthenopteron 2 (Fig. 276), Upper Devonian of Scaumenac 



1 Traquair, Trans. Roy. Soc. Edinb. xxvii. 1875, p. 383. 

 - Whiteaves, Trans. Roy. Soc. Canada, vi. 188b, p. 77. 



