The Crossopterygii 



6oi 



Fig. 371.— Arm of a frog. 



this the basal segment of the archipterygium gives place to 

 the humerus, the diverging segments seen in the most special- 

 ized type of archipterygium (Polypterus) become perhaps radius 

 and ulna, the intermediate quadrate mass of cartilage possibly 

 becoming carpal bones, and from these spring the joints called 

 metacarpals and phalanges. In the Amphibians and all higher 

 forms the shoulder-girdle retains its primitive insertion at a 

 distance from the head, and 

 the posterior limbs remain 

 abdominal. 



The Amphibians are there- 

 fore primarily fishes with 

 fingers and toes instead of 

 the fringe -fins of their an- 

 cestors. Their relations are 

 really with the fishes, as 

 indicated by Huxley, who 

 unites the amphibians and 

 fishes in a primary group, 

 Ichthyopsida, while reptiles 

 and birds form the contrasting group of Sanropsida. 



The reptiles differ from the Amphibians through accelera- 

 tion of development, passing through the gill-bearing stages 

 within the egg. The birds bear feathers instead of scales, 

 and the mammals nourish their young by means of glandular 

 secretions. Through a reptile-amphibian ancestry the birds 

 and mammals may trace back their descent from palaeozoic 

 Crossopterygians. In the very young embryo of all higher 

 vertebrates traces of double-breathing persist in all species, 

 in the form of rudimentary gill-slits. 



The Fins of Crossopterygians. — Dollo and Boulenger regard 

 the heterocercal tail as a primitive form, the diphycercal form 

 being a result of degradation, connected with its less extensive 

 use as an organ of propulsion. Most writers who adopt the 

 theory of Gegenbaur that the archipterygium is the primitive 

 form of the pectoral fin are likely, however, to consider the 

 diphycercal tail found associated with it in the Ichthyotomi, 

 Dipneusti, Crossopterygii as the more primitive form of the tail. 

 From this form the heterocercal tail of the higher sharks and 



