viii. 2 PALAEONISCIDS 233 



*Cleithrulepis in the Triassic. Similar forms arose again later among 

 the holosteans and teleosteans and we have therefore evidence that 

 this type of animal organization tends to evolve into deep-bodied 

 creatures. *Dorypterus further resembles modern teleosteans in a great 

 reduction of its scales and in the forward movement of the pelvic fins. 



Towards the end of the Triassic animals of typical palaeoniscid 

 type became rare; they were replaced by their more active and speedy, 

 mainly marine descendants, the Holostei (p. 234). Certain of the lines 

 that branched off in the Palaeozoic have, however, survived to the 

 present time, and in spite of subsequent specializations they give us 

 some idea of the characteristics of these early Actinopterygii. Perhaps 

 the most interesting of these survivals are Polypterus, the bichir, and 

 the related Calamoichthys, both inhabiting rivers in Africa. The air- 

 bladder shows some similarity to a lung. It forms a pair of sacs lying 

 ventrally below the intestine and opening to the pharynx by a median 

 ventral 'glottis' (Fig. 157). This is the arrangement found in lung- 

 fishes (except Ceratodus) and in tetrapods, and it seems reasonable to 

 suppose that it has survived in Polypterus from Palaeozoic times. 

 However, it is not certain to what extent the air-bladder is still used 

 as a lung, for Polypterus cannot survive out of the water. 



This fish shows many other ancient characteristics. The covering 

 of thick rhomboidal scales, hardly overlapping, gives the animal an 

 archaic appearance; the structure of the scales is 'palaeoniscoid'. In 

 the skin there is a layer of denticles outside the scales. The presence 

 of a spiracle, the arrangement of the skull bones, and many other 

 features suggest that Polypterus is essentially a palaeoniscid surviving 

 to the present day. In the intestine there is a spiral valve, which 

 appears to have been present in the early Crossopterygii and Actino- 

 pterygii (as judged from fossilized 'coprolites') and occurs today not 

 only in the Dipnoi but also in sturgeons and, though much reduced, in 

 Lepisosteus and Amia. There is a single pyloric caecum in Polypterus 

 (the caeca are well developed in sturgeons, Lepisosteus, and Amid). 

 The tail of Polypterus is no longer markedly heterocercal, but shows 

 distinct signs of that condition. We can even find a parallel among 

 Carboniferous palaeoniscids for some of the special features of Poly- 

 pterus. The long body and dorsal fin are found in the fossil *Tarrasius, 

 which may have been close to the ancestry of Polypterus, though it 

 lacks the covering of scales. The pectoral fin in *Tarrasius, as in 

 Polypterus, has a peculiar lobed form, which has been compared with 

 the 'archipterygial' pattern (p. 269) and hence held to show that 

 these animals are related to the Crossoptergyii. The resemblance is, 



