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 ARCH^EORNITHES or SAURUR^E. Ancient extinct birds, 

 connecting Birds and Reptiles 



The oldest known bird is Archceopteryx, 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 a crow. 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 vertebrae 

 of the long tail bears a pair of lateral rectrices a unique arrangement. 

 There .is no pygostyle. The vertebras 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 Opisthocomus are the three digits of the 

 fore-limb clawed ; in most cases claws are confined to the thumbs. 

 Caudal vertebrae are apparently not more than thirteen in number. 



I. Division RATIT^E. Running Birds with raft-like unkeeled 

 breast-bone 



The African ostrich (Struthio) 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 caeca 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. 



