x TELEOSTEI, CROSSOPTERYGU 365 



were one of the predominant groups of fishes but are now on the verge 

 of extinction, being represented by only a couple of genera Polypterus, 

 found in the great river-basins of Africa which drain into the Atlantic 

 and Mediterranean, and Calamichthys which is restricted to the rivers of 

 tropical West Africa. 



Polypterus (Fig. 152) is a graceful fish, perhaps the most conspicuous 

 feature of which is its coating of rhomboidal scales, closely fitted together, 

 each covered on its surface with a shining layer of enamel-like modified 

 dentine to which it is fashionable, though in the present writer's opinion 

 unnecessary, to apply the special term ganoin. Such scales are known 

 technically as ganoid. The fins, both median and paired, show charac- 

 teristic peculiarities. The former, at first continuous along the dorsal 

 side of the body and round the tip of the protocercal tail (Fig. 155, D), 

 becomes in its dorsal portion divided up into a number of separate 



olfl 



cif 



FlG/152. 



Polypterus. a.f, anal fin ; d, dorsal fins ; olf.i and 2, olfactory openings ; op, opercular 

 opening ; pl.f, pelvic fin ; v.c.l, spiracle. 



small dorsal finlets (Fig. 152, d) : hence the generic name Polypterus. 

 The pectoral fins are again of a special type known as crossopterygian 

 from the fact that the thin portion of the fin forms a kind of fringe 

 round the edge of a thick, fleshy, scale-covered basal lobe in contrast 

 with the aetinopterygian type seen in the great majority of fishes where 

 there is no such fleshy lobe. 



As regards the alimentary canal the first thing to notice is that the 

 mouth opening has in the adult, as is the case in the Teleostei, become 

 shifted from the ventral side of the head to the tip of the snout. Into 

 the buccal cavity there opens in the middle line of its roof a gland which 

 can be recognized as the pituitary body. This therefore retains a very 

 primitive condition in Polypterus. The branchial apparatus resembles 

 in its main features that of a teleost but on the other hand it retains the 

 primitive feature that the spiracle is still an open cleft. Its outer narrow 

 slit-like and valvular opening may be seen on the dorsal surface of the 

 head (Fig. 152, v.c.T). 



