VIII. 2 



PALAEONISCIDS 229 



clumsy, heavily armoured creatures. From this early type many lines 

 have been derived and can be followed with some completeness to 

 their extinction or modern descendants. Various classifications have 

 been suggested. The one used here is simple but for that very reason 

 obscures the multiplicity of parallel lines. A recent classification 

 recognizes fifty-two orders of Actinopterygii (Grasse). 



d. 



Fig. 145. Scales of some early fishes. 

 A, hypothetical condition with denticle-like substance (d.) attached to a basal bony plate lying 

 in the connective tissue (ct:)\ B, 'cosmoid' scale of early crossopterygians, showing the cosmine 

 layer (co.) ; epidermis (ep.) ; vascular canals (hv.) and underlying 'isopedin' {is.); C, palaeoniscoid 

 scale with layers of 'ganoin' (ga.); d, lepidosteoid scale of the gar-fish with tubules (/.). (From 

 Goodrich, Vertebrata, A. & C. Black, Ltd.) 



The Devonian and Carboniferous forms are grouped together in 

 the order Palaeoniscoidei, and animals of similar type survive today 

 as Polypterus, the bichir of African rivers, which though showing some 

 specializations remains in its general organization near the palaeoniscid 

 level. 



A typical Palaeozoic palaeoniscid such as *Cheirolepis was a long- 

 bodied creature (Figs. 146 and 147) with a heterocercal tail, single 

 dorsal fin, and pelvic fins placed far back on the body. The pectoral 

 and pelvic fins had broad bases and the radials fanned out from a small 

 muscular lobe, present in all early actinopterygians but lost in later 

 forms. The body was covered with thick rhomboidal scales very 

 similar to those of acanthodians. They articulated by peg and socket 

 joints and have a structure known as palaeoniscoid (Fig. 145). The 

 scale is deeply embedded and grows by addition both to the bony or 

 isopedin portion and to the shiny surface-layer, the ganoin, which 

 thus becomes very thick. There is a middle layer of 'pulp' correspond- 

 ing to the cosmine layer of the cosmoid scale of Crossopterygii 



