MAMMALIA. 273 



CHAPTER IX. 

 Class V. MAMMALIA.* 



Warm-Hooded, hairy animals with double occipital condyle. They 

 are viviparous and suckle their young with the secretion of milk 

 (mammary} glands. 



As opposed to Birds, Mammals are adapted, by the similar struc- 

 ture of the two pairs of extremities, to live principally on land. 

 There are, however, in this class also forms which are fitted in 

 various degrees for an aquatic life, and even live entirely in water, 

 and again forms which move and find their food in the air. 



The surface of the skin is rarely quite smooth as in the Cetacea, 

 but is traversed by numerous curved, spiral, and partly crossing 

 furrows, and in many places (sole of foot, ischial callosities) is thick- 

 ened and indurated, so as to form firm, horny plates. 



The hairy covering is to Mammalia (named " Haarthiere " by Oken), 

 what the plumage is to Birds. Hairs are never entirely absent: 

 even the huge aquatic forms and the largest of the tropical terrestrial 

 species which seem to be naked, possess hairs on certain parts of the 

 body ; e.g., the Cetacea have short bristles, at least on the lips. Hairs, 

 like feathers, are epidermal structures (fig. 666.) The bulbous root is 

 placed on a vascular papilla (pulpa), at the bottom of a pit, which 

 projects into the cutis and is lined by epidermal cells (hair-follicle) 

 while the upper part, or shaft, projects freely on the surface of the 

 skin. Two kinds of hairs may be distinguished, according to the 

 strength and rigidity of the shaft, viz., contour hairs and woolly 

 hairs. Woolly hairs are delicate and curled, and surround in larger 

 or smaller numbers the base of each contour hair. The finer and 

 warmer the fur, the more numerous are the woolly hairs (winter-fur). 

 When the contour hairs have a greater strength they become bristles, 



* Job. Ch. D. v. Schreber, "Die Saugethiere in Abbilduugen nach tier Natur 

 mit Beschreibungen, fortgesetzt von Job. Andr. "Wagner," JBd. I. VII., und 

 Suppl. I. Y. Erlangen und Leipzig, 1775 1855. 



E. G-. St. Hilaire et Fred. Cuvier, " Histoire naturelle des Mammiferes," 

 Paris, 18191835. 



C. J. Temmink, " Monographic de mammalogie." Leiden, 1825 1841, 



R. Owen. " Odontography," 2 vol. London, 18401845. 



Blasius, " Die Saugethiere Deutschlands " 1875. 



G. Giebel, " Die Saugethiere in zoologisch-anatomischer und palaontologischeM 

 Hinsicht." Leipzig, 1850. 



A. E. Brehm, i; Illustrates Thierleben " I., II., und III. 



And. Murray, "The Geographical Distribution of Mammalia." London. 1866. 



VOL. II. 18 



