14 VERTEBRATE ANIMALS OF THE UNITED STATES 



1. Sauropsida. — Vertebrates which produce a very large egg and 



are either oviparous or ovoviviparous; which have a single 

 median occipital condyle, and epidermal coverings in form 

 of scales or feathers (Reptiles and Birds). 



2. Mammalia. — Vertebrates which produce a minute egg (except 



the Monotremata), and are viviparous with uterine embryonic 

 development; which nourish the young by means of mam- 

 mary glands; which have a pair of occipital condyles, and an 

 epidermal covering in form of hair (Mammals). 



Key to the Classes of Vertebrates 



ai Aquatic vertebrates whose locomotory appendages are fins i. Fishes. 



a.2 Aquatic or land vertebrates with legs or wings as locomotory 

 appendages, or without them, 

 bi Body naked (in American species), being without special 



integumentary coverings; toes without claws or nails 2. Amphibians 



b2 Body not naked, but with special integumentary coverings; 

 toes with claws or nails. 



Ci Body covered with horny scales or plates 3. Reptiles. 



Co Body covered with feathers 4. Birds. 



C3 Body covered with hair 5. Mammals. 



