352 CLASSIFICATION OF VERTEBRATES. 



The Euornithes date back no farther than the eocene, in which period 

 representatives of the cormorants, pelicans, flamingoes, falcons, kingfishers, 

 and pheasants appear. The colics, oilbirds, and opisthocomi are not known 

 .as fossils, while the other groups appear with the miocene. 



CLASS II. MAMMALIA. 



Hair-bearing amniotes with two occipital condyles ; lower 

 jaw suspended directly from the cranium without the interven- 

 tion of the quadrate ; ankle joint between the tibia and fibula 

 and the first row of tarsal bones ; brain with well-developed 

 corpus callosum ; a complete diaphragm ; heart four-chambered ; 

 only one (left) aortic arch persisting ; red blood corpuscles 

 non-nucleate, usually circular in outline ; eggs (except in 

 monotremes) minute, and undergoing a total segmentation, 

 the embryonic development taking place inside the mother ; the 

 young, when born, nourished by milk secreted by the mammary 

 .glands of the mother. 



The skin in the mammals has a well-developed stratum 

 -corneum (p. 88), which is never molted as a whole, as in the 

 reptiles and lower vertebrates, but comes away piecemeal. 

 The skin is as a rule pigmented, and the pigment may occur 

 in either the deeper (dermal), or more superficial (epidermal) 

 portions. The epidermis gives rise to various structures, the 

 most noticeable and most characteristic of which is hair, the 

 structure and development of which is described elsewhere 

 (P- 97)- The hair is usually abundant, and covers most of the 

 body. The other extreme is reached in the cetacea, where it 

 may be reduced to from two to eight pairs of bristles in the 

 mouth region, these occurring in some cases only in foetal life. 

 Frequently one can distinguish two kinds of hair, one straight 

 and stiff, covering a deeper woolly hair. Hair can undergo con- 

 siderable modifications. It may be straight or curly ; it may 

 develop into bristles, or even into strong protective spines, as in 

 .the porcupines, etc. Frequently certain hairs about the mouth 

 (vibrissae) have tactile functions, their roots being enveloped in 

 a rich plexus of nerve fibres. In some cases the hair seems to 

 persist throughout life (tail and mane of horses), but usually it 

 falls out and is replaced by new hair, this molting occurring 



