THE PROGRESS OF LIFE 85 



convex behind. The inner opening of the nostrils 

 has also gradually moved backwards, from the middle 

 of the roof of the mouth in the early forms, to the 

 base of the skull in living forms. 



Archeopteryx, from the upper Jurassic of Bavaria, is 

 the earliest known bird. At first it was thought to be 

 a reptile, because it has three free fingers with claws on 

 each hand, biconcave vertebrae, a long lizard-like tail, 

 abdominal ribs, and teeth in its jaws. Also the bones 

 of the pelvis are not united together, and even the 

 metatarsi of the leg seem to be but imperfectly joined ; 

 all of which are reptilian characters. Nevertheless, 

 the presence of feathers on the wings and tail, the 

 structure of the foot, and the fact that all the bones 

 of the skull are fused into one, are such truly avian 

 characters that all naturalists now agree that it should 

 be considered as a bird. Only two specimens are 

 known. In the middle Cretaceous of North America 

 the remains of other birds have been found, all of 

 which had teeth in their jaws. There were two 

 distinct types, one of which IchtJiyornis had the 

 teeth in separate sockets and biconcave vertebrae, while 

 the other Hesperornis had the teeth in a single 

 groove in each jaw, and the centra of the vertebras 

 were saddle-shaped, as in ordinary birds. Hesperornis 

 was a swimming bird, which had lost the power of 

 flying, and its wings were in a very degenerate state ; 

 but Ichthyornis must have been a powerful flyer, ap- 

 parently feeding upon fish. In the upper Cretaceous 

 of North America remains of other birds have been 

 found, some of which may belong to the living group 

 of carinate birds. 



For a long time it was thought that mammals 



