370 AN INTRODUCTION TO THE STUDY OF FOSSILS 



pression of the breastbone, or back, or both. The lungs extend 

 as prolongations all over the body and into many of the hollow 

 bones. Birds thus agree with insects, the only other typically 

 aerial class in having the fresh air carried throughout a large por- 

 tion of the body and not only into the chest. The brain is large. 



Fig. i6o. — The earliest known bird, — Aycheopteryx, of the size of a small crow, 

 from the Upper Jurassic of Solenhofen, Bavaria. The jaws bear many sharp, 

 conical teeth (there are 26 in the upper jaw). A, right hand (or wing bones). 

 B, right foot. C, restoration, ra., carpus (not well known); da., claws; /., dis- 

 tinct feathers, — each pair of these tail feathers is attached to a separate vertebra ; 

 m.ca., metacarpal (palm) bones; m.la., tarso-metatarsals ; the fusion of the 

 metatarsals is not as complete as in modern birds; ph., phalanges; ra., radius; 

 ul., ulna; I, II, III, IV, digits, ist (corresponding to thumb and great toe), 

 2d, etc. {A from Osborn, after Dames; B from Osborn, after Owen ; C modified 

 from Woodward's "Vertebrate Paleontology," after Pycraft.) 



The heart is completely four-chambered. The sense of smell 

 is poorly developed, but those of sight and hearing are usually 

 remarkably acute. Birds are oviparous. As the ovum or 

 yolk passes dow^n the oviduct it receives first the coat of white, 

 or albumen, next a parchment-like membrane and finally a 

 calcareous coating, the shell, over all. 



Migration may have been impressed upon the northern birds 



