47 8 FISHES CHAP. 
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 gephyrocereal. Teeth with the external enamelled layer 
of dentine infolded towards the axis in the form of radially 
. 
Fic. 274.—Skull of a Rhizodont (Rhizodopsis sauroides), Lower Carboniferous. A, 
lateral view; B, the dorsal surface; and C, ventral view. an, Angular; d, 
dentary ; f, frontal ; id, infra-dentary ; 7, principal jugular plates ; Jj, lateral 
jugulars ; m, mandible; m.j, median jugular; mz, maxilla; 0, orbit; op, oper- 
culum ; p, parietal ; p.f, post-frontal ; p.m, premaxilla ; p.op, preoperculum ; so, 
suborbital ; s.op, suboperculum ; sg, 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 :— 
Lhizodus, Lower Carboniferous of Scotland and Northumberland ; 
Tristichopterus' (Fig. 275), Old Red Sandstone of Scotland ; 
Eusthenopteron® (Fig. 276), Upper Devonian of Scaumenac 
1 Traquair, Trans. Roy. Soc. Edinb. xxvii. 1875, p. 383. 
* Whiteaves, Trans, Roy. Soc. Canada, vi. 1888, p. 77. 
