29: 



P0LYPTER1NI 



like that of the Actinopterygii, with a very small lobe. The tail is 

 outwardly diphycercal (p. 104). The median dorsal fin, continuous 

 in the larva with the epichordal lobe of the tail [422&, 67], becomes 

 broken up in the adult into a series of finlets of peculiar structure 

 (Figs. 264, 272) (Steindachner, Traquair [441 J). Each is supported 

 in front by a strong movable elongated scale of paired origin, bear- 

 ing at its distal end a number of normal lepidotrichia. There is 

 an anal fin. The body is covered with thick rhomboid shiny scales, 

 with a small articulating superior and a larger anterior process 

 (Fig. 264). The scales differ markedly in their finer structure 

 from those of the Osteolepidoti, and are built on essentially the 

 same plan as the true ganoid scales of the Actinopterygii (p. 217). 

 They consist of three principal layers passing into each other at 



Fi< 



VC. 



Portion of a thick transverse section of the scale of Polypterus bichir, Geottr., much 

 enlarged, b, inner bony j»r isopedine layer ; e, canaliculi of the cosmine layer ; <l, superficial 

 denticle; i/, ganoine layer; h, system of horizontal vascular canals; o, opening of vertical 

 canal on outer surface ; v.c, vertical canal. 



the sides : bony lamellae below, ganoine lamellae above, and a 

 layer with vascular canals and dentinal tubules between these two 

 (Fig. 265). The dentinal tubules are chiefly, if not exclusively, 

 given oft' by fine canals derived from the vascular channels ; this 

 intermediate layer obviously corresponds to the middle layer in the 

 scales of the Palaeoniscids (Fig. 288). The scales grow by the 

 addition of concentric layers, and are, in fact, typical ganoid scales 

 (p. 218). Moreover, to their outer surface are generally fixed small 

 sharp denticles, sometimes scattered over the whole exposed surface 

 of the scale, more often restricted to its free edge. The lepido- 

 trichia are of the same structure (Fig. 184), and also the dermal 

 bones of the shoulder-girdle and skull, but on the latter the denticles 

 are absent. 



The skull has no pineal foramen ; the orbits are small and far 

 forward. The double nares are on the dorsal surface of the snout,. 



