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 colies, oilbirds, and opisthocomi are not known 

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



Class II. IMammalia. 



Hair-bearing amniotes with two occipital condyles ; lower 

 jaw suspended directly from the cranium without the inter\'en- 

 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 ; 

 onl}- one (left) aortic arch persisting ; red blood corpuscles 

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

 Tiionotremes) minute, and undergoing a tcital segmentation, 

 the embryonic de\"elopment taking place inside the mother ; the 

 3'oung, when born, nourished by milk secreted by the mammary 

 •glands of the mother. 



The skin in the mammals has a well-developed stratum 

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

 Teptiles 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 

 ^nay be reduced to from two to eight pairs of bristles in the 

 mouth region, these occurring in some cases onlv 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 

 (vibrissse) 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 



