686 BIRDS. 
About the twentieth day the beak, which has a hard “‘ tooth” on the 
tip, perforates the membranes of the air-chamber, and the air, rushing 
in, expands the hitherto functionless lungs. At the same time import- 
ant changes occur in the circulatory system, ‘‘the umbilicus becomes 
completely closed, the allantois shrivels up, and the chick, piercing 
the broad end of the shell with repeated blows of its beak, steps out 
into the world.” 
CLASSIFICATION OF BIRDS 
I. Sub-Class ARCHAORNITHES or SAURUR4. Ancient extinct birds, 
connecting Birds and Reptiles 
The oldest known bird is Archeopteryx, two specimens of which 
have been found in the Solenhofen Lithographic Stone (Upper Jurassic) 
of Bavaria. ‘‘The stone is so fine-grained that, besides the bones of 
the wings, the furculum or merrythought, the pelvis, the legs, and the 
tail, we have actually casts or impressions on the stone (made when it 
was as yet only soft mud) of all the feathers of the wings, and of the 
tail.”,—Nicholson and Lydekker. 
This link between Birds and Reptiles seems to have been a land bird 
about the size of acrow. The skull is like that of a typical bird. The 
upper jaw shows thirteen pairs of conical teeth, the lower about three 
pairs. They are embedded in sockets. Each of the twenty vertebrze 
of the long tail bears a pair of lateral rectrices—a unique arrangement. 
There is no pygostyle. The vertebrae seem to have been either 
amphiccelous or with flat ends; the ribs are very slender, without 
uncinate processes ; there seem to have been ‘‘abdominal ribs” ; the 
sternum is not clearly known; there is a U-shaped furcula. The 
metacarpals seem to have remained separate; the first finger has 
two phalanges, the second three, the third three or four, and all are 
clawed. There is a tarso-metatarsus and four toes, as in the pigeon. 
II. Sub-Class NEORNITHES 
The metacarpals are fused. The second finger is the longest, and 
the third is reduced. Only in Opzsthocomus are the three digits of the 
fore-limb clawed; in most cases claws are confined to the thumbs. 
Caudal vertebrz are apparently not more than thirteen in number. 
1. Division RaTIT#. Running Birds with raft-like unkeeled 
breast-bone 
The African ostrich (S¢vuthzo) is represented by two or three species, 
at home in the plains and deserts of Africa, and notable for their size, 
swiftness of foot, and beauty. There are but two toes, the third and 
the fourth, with stunted nails. There are no clavicles. The pubes 
form a ventral symphysis. The enormous size of rectum and czca is a 
unique character. The ostrich is polygamous, and at the breeding 
season the hens lay the eggs, at intervals, in a hollow dug out in the 
sand by the male. The eggs are incubated by both parents, but 
especially by the cock. 
