CHORDATA — VERTEBRATA — MAMMALS 373 



the young Hoactzin, as in Archeopteryx, the hand is longer 

 than the forearm, but before the resorption of the claws the 

 hand ceases to grow while the forearm becomes much longer, so 

 that their relative lengths become reversed. 



Birds and dinosaurs may have descended from a common stem 

 derived from the Rhynchocephalia. 



1. Give distinguishing characters of Class Aves. 



2. Distinguish the ankle-joint of reptiles and birds from 

 that of mammals. 



3. What is the significance of a keel to the breastbone ? 



4. Why do grain-eating birds swallow small stones ? What 

 difference between the gizzard of these birds and that of flesh- 

 eaters ? 



5. What is the principal difference between the breathing 

 apparatus of birds and that of mammals ? 



6. What is the possible origin of migration among northern 

 birds ? 



7. Give the geologic range of birds. 



8. Why are birds rare as fossils ? 



9. Describe Archeopteryx, distinguishing the reptilian from 

 the avian characters. 



10. Under what conditions were the lithographic slates of 

 Solenhofen deposited ? 



11. What is the probable origin of birds ? 



12. In what ways do modern birds and especially the Hoactzin 

 indicate their ancestry in their ontogeny ? 



Class G, Mammalia (Mammals) 



Air-breathing, warm-blooded vertebrates usually with a 

 protective exoskeleton of hair. The temperature of most 

 mammals is about 98° F. The skull is articulated with the 

 backbone by two rounded prominences (condyles), instead of 

 by one as in birds and in most reptiles, and the lower jaw is 

 articulated with the skull directly without the aid of the separate 

 quadrate bone present in those two classes. The teeth, typi- 

 cally forty-four in the placental mammals, are nearly always 

 differentiated into three incisors on each side above and below, 



