664 APPENDIX B 



trachea with the two bronchi, taking the place of the larynx as a sound-producing 

 structure; skeleton made rigid by fusion of many of its bones, the breastbone 

 in all strong fliers enlarged for insertion of massive flight muscles; body stream- 

 lined; a single left ovary and oviduct retained in most forms; eggs and egg mem- 

 branes essentially reptilian, but the eggs usually incubated by the heat of the 

 parent's body, and parental care highly developed in the majority of modern 

 forms. 



The birds form a large and highly successful group, which is apparently not 

 even yet at its zenith. Relationships among modern birds are so close and the 

 assemblage is so homogeneous that it is not easy to subdivide the existing species 

 into numerous well-defined groups. The differences that distinguish the orders 

 and families are much less pronounced than in the other classes of vertebrates. 



Class 8. The Mammals, or Hairy Vertebrates (Mammalia). Warm-blooded, air- 

 breathing vertebrates with bodies more or less covered with hair. The skin con- 

 tains sweat, oil, and mammary glands, the secretion of the latter (milk) serving 

 to nourish the young. The skull has two occipital condyles; the lower jaw consists 

 of a< single bone, the dentary, which articulates with the squamosal bones; the 

 articulate and quadrate bones of the ancestral jaw hinge are transformed into 

 the malleus and incus bones of the middle ear, the third ear bone (stapes) being 

 inherited from the reptiles. The teeth are imbedded in sockets in the jawbone 

 and are of two sets (milk teeth and permanent teeth) ; they are differentiated into 

 incisors, canines, premolars, and molars. The ear is usually furnished with an 

 external trumpet, the tympanic membrane lies at the bottom of a tube, and the 

 cochlea of the inner ear (the auditory sense organ) is spirally coiled except in the 

 monotremes. There are fleshy cheeks and lips covering the edges of the jaws and 

 teeth (replaced by horny plates in monotremes). 



The neck nearly always has only seven vertebrae; the centra of the vertebrae 

 are separated by cartilaginous disks ; usually the ribs articulate with the vertebrae 

 by two heads ; the first digit of f orelimbs and hind limbs has two bones, all the re- 

 maining digits three (a lower number than in the reptiles). The thoracic and ab- 

 dominal cavities are separated by a muscular diaphragm that aids in breathing. 

 The brain is relatively large, the cerebrum and especially the cerebral cortex (neo- 

 pallium) greatly enlarged, and the two cerebral hemispheres are connected by the 

 corpus callosum, a massive bundle of fiber tracts (rudimentary in marsupials and 

 absent in monotremes) . The cerebellum is large, complex, and solid. 



A cloaca (combined anal and urinogenital opening) is absent except in mono- 

 tremes and some insectivores; a urinary bladder is present. A penis is always pres- 

 ent. The eggs are alecithal, microscopic (except in monotremes) ; they develop in 

 a uterus (except in monotremes), to the walls of which the embryo is attached 

 by a placenta (except in the monotremes and most marsupials) ; the young are 

 "born alive" (except in monotremes). 



The classification of the mammals is given below in somewhat more detail 

 than was done for the other classes of vertebrates, with special attention to the 

 primates. 



Class Mammalia 



Subclass I. THE EGG-LAYING MAMMALS. (Prototheria or Monotremata.) 

 Includes the duckbill (Ornithorhynchus) and spiny anteater (Tachyglossus). 



