The Crossopterygii 227 
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, Fie. 160.—Arm of a frog. 
Ichthyopsida, while reptiles 
and birds form the contrasting group of Sauropsida. 
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 palzozoic 
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 Ichthyotomz, 
Dipneusti, Crossopterygii as the more primitive form of the tail. 
From this form the heterocercal tail of the higher sharks and 
