64 



in Ganoids and Teleosteans is mainly hydrostatic in function, 

 into an accessory organ of respiration, which corresponds to 

 the lungs of higher Vertebrata. 



One of the most important changes which took place in 

 the series of ancestral forms leading upwards from the top 

 of the Proto-Ganoidei was the conversion of the ichthyoptery- 

 gium, or fin-like limb, found in Fishes, into the cheiroptery- 

 gium, or true limb, characteristic of higher Vertebrates. 

 This change was probably effected by the elongation of the 

 basal cartilage (metapterygium) of the fin to form the 

 primitive humerus or femur, which became directed outwards 

 from the body instead of lying more or less parallel with it. 

 The other skeletal elements of the cheiropterygium are 

 probably derived from modified fin rays.''' The group in 

 which this higher form of limb was first developed is 

 Balfour's Proto-Pentadactyloidei. This must have split up 

 into two lines of descent; the one forming the common 

 ancestors of the Amphibia and the Labyrinthodontia, and the 

 other leading upwards to higher forms — the Proto-Amniota. 



The Urodelous Amphibia probably represent with com- 

 paratively little modification the first of these two ancestral 

 lines. Lungs had now become completely developed, and 

 their establishment as the chief respiratory organs had 

 caused changes in the course of the circulation and in the 

 structure of the heart. The Anura are a somewhat more 

 modified group which have sprung from the Urodela, while 

 the Gymnophiona are clearly degenerate forms, which prob- 

 ably diverged from the ancestral Urodela. The extinct 

 Labyrinthodontia may be regarded as a lateral ofi'shoot from 

 the primitive Amphibians. 



The Proto-Amniota divided into two great series of 

 ancestral forms, the primitive Sauropsida leading upwards to 

 Reptiles and Birds, and the early Mammalia. Just below 

 * Comp. EmbryoL, v. ii, p. 510. 



