408 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



The members of the only two existing genera of crossopterygians, 

 Polypterus and Calamoichthys, live in the rivers of tropical Africa in 

 legions subjected to the usual tropical alternation of wet and dry seasons, 

 but do not show any adaptation to a life out of the water or even an un- 

 usual capacity to use the air-bladder as a respiratory organ, though, ac- 

 cording to Budgett, the air-bladder of Polypterus is an accessory respira- 

 tory organ rather than a hydrostatic organ. 10 



Observations on Polypterus bichir show that the pectoral fins are used 

 for progression, but their primary function is to act as balances, and they 

 exhibit the characteristic trembling movements so often seen in the bal- 

 ancing fins of teleosts. In captivity the fish often remains motionless 

 for long periods of time at the bottom of the water, the anterior part of 

 the body resting on the tips of the pectoral fins. The latter are much 

 less elongated than the notable "fringed limbs" of certain Devonian 

 forms — such, for example, as Holoptychius. The pelvic fins of Polyp- 

 terus are still less developed, constituting nothing more than spines. 



Calamoichtliys, the other genus of living crossopterygians, is a very 

 agile fish, but is snakelike in form, swimming like a snake, and its body 

 fins are still more reduced. In these respects it is clearly degenerate. 



In the opinion of most specialists, the crossopterygians occupy a cen- 

 tral position among fishes. Eemotely connected with the selachians on 

 the one hand and more intimately connected with modern bony fishes on 

 the other, they probably also represent the ancestral stock from which 

 the stegocephalian amphibians and the dipnoans have had their origin. 

 Although the group is regarded as ancestral to amphibians, it is to be 

 noted that the lingering representatives are not by any means as am- 

 phibious in their habits as are the remaining dipnoans. Nevertheless 

 both orders possess swim-bladders which are primarily lungs, connected 

 with the oesophagus by a duct. The living crossopterygians show indeed 

 a paired lung, though it is without internal sacculations. As seen in the 

 living dipnoans, the inside of the swim-bladder is highly sacculated, in- 

 creasing its respiratory value. In Ceratodus the lung is single, but in 

 Protopterus and Lepidosiren the organ is double, large, and quite lung- 

 like in character. It is seen that minor changes, such as could well exist 

 within a single family, might show a much larger use of the lungs in the 

 crossopterygians. 



There is reason to believe that the modern representatives of both of 

 these groups are degenerate in certain respects. This is a feature which 

 often marks the lingering representatives of archaic types, no longer con- 

 testing for supremacy in the living world. Dollo, in fact, has shown 



18 Largely from G. W. Bridge : Fishes. The Cambridge Natural History Series, vol. 

 vii. 



