CHONDROSTEI 309 



Coccolepis, however, they only slightly exceed the radials in number 

 (A. S. Woodward [501]). The tail is typically heterocercal, and 

 generally distinctly forked owing to the great development of the 

 anterior region of the hypochordal lobe. Along the tipper edge of 

 the dorsal lobe (Fig. 280) runs the vertebral column, tliQ^pichordal 

 fin being represented externally only by the row of fulcra it is this 

 which distinguishes the heterocercal tail of the Chondrostei from 

 that of other fish. 



There is no interoperculum, and no large paired inferior gulars, 

 although in Palaeoniscids the first pair of lateral gulars may be 

 enlarged. The anterior median gular is small or absent. The scales 

 vary from the rhombic to the cycloid type, or persist as large plates. 

 The articulating peg projects from the dorsal edge. They ofteir 

 differ considerably in structure on various parts of the body, and it is 

 characteristic that while on the trunk the scales are disposed in 

 the usual oblique rows bending backwards and downwards, this 

 disposition is suddenly reversed on the tail, where they pass down- 

 wards and forwards (Fig. 290). Elongated rhomboid scales are 

 preserved on the tail even when they are lost on the trunk. 



TKe frequent presence of a hyoid demibranch, of a spiracle, 

 and of a spiracular pseudobranch, indicates primitive structure. 

 It has been noticed above that the brain shows certain primitive 

 characteristics ; the same may be said of the urinogenital organs 

 and other viscera. 



Sub-Order 1. PALAEONISCOIDEI. 



In which the scales are of the palaeoniscoid type of structure 

 (p. 218), with ganoine and cosmine (Figs. 191, 288). 



Family PALAEONISCIDAE. These are the most primitive and earliest 

 of the Actinopterygii. They appear in the Lower Devonian, are abundant 

 in the Carboniferous and Permian, and die out in the Jurassic times. 

 The dermal skeleton is covered with ganoine. The scales are usually 

 rhomboid, with articulating pegs ; but in some (Coccolepis, Cryphiolepis) 

 the scales are cycloid and deeply imbricating on the trunk, and in 

 Phanerosteon the trunk is almost scaleless. 



The orbits are far forward and the snout blunt, so that the nostrils 

 are lateral, or even ventral, rather than dorsal. The superficial cranial 

 bones differ but little in plan from those of the normal Teleostome. The 

 opercular, subopercular, and lateral gulars form a very continuous series. 

 The preopercular is narrow and bent forwards, the maxilla spreads back- 

 wards over the cheek ; the preniaxilla is small ; a large median ethmoid 

 probably includes the nasals. The orbit is surrounded by a narrow ring 

 of few bones. The supratemporals are a narrow row of transversely set 

 bones ; the post-temporal is large (Figs. 285-7). 



