CLASS MAMMALIA 411 



Subclass II. Eutheria (u tlie' rl a; G., eii, true, and therion, mammal). — Viviparous 

 forms, including all the rest of the mammals. 

 Division I. Didelphia (dl del' fl a; G., dis, double, and delphys, uterus). — Form 

 no true placenta and carry the young in a pouch. 

 Order 2. Marsupialia. 

 Division II. Monodelphia (m6n 6 del' fl a; G., monos, single, and delphys, 

 uterus). — Placental mammals, which form a true placenta by means of 

 which the young are nourished and which never carry the young in a pouch. 

 Section I. Unguiculata (tin gwlk u la' ta; L., unguiculatus, provided with 

 claws). — Clawed mammals. 

 Orders 3 to 10. Include Insectivora, Chiroptera, Carnivora, Rodentia, and 

 Edentata. 

 Section II. Primates (as a zoological group name, pri mil' tcz; L., primatis, 

 one of the first in rank). — Contains mammals with nails on the fingers 

 and toes. 

 Order 11. Primates. 

 Section III. Ungulata (tin gu la' ta; L., ungulatus, provided with hoofs). — 

 Hoofed mammals. 

 Orders 12 to 16. Include, besides the typical hoofed forms, the Sirenia. 

 Section IV. Cefacea (se ta' she a; G., cetos, whale). — Mammals without limbs. 

 Orders 17 to 18. Include whales and other similar types. 



In this arrangement those mammals which are most primitive are 

 placed first and those which are most highly modified last. This brings 

 the primates and man in the middle of the series since they are neither 

 the most primitive nor the most highly modified. 



439. Origin of Mammals. — There are many resemblances between 

 mammals and certain primitive reptiles known as cynodonts, the fossil 

 remains of which have been found in South Africa. These resemblances 

 point so clearly to relationship between the two groups that the cynodonts 

 are now generally considered the ancestors of the mammals. 



440. Monotremes. — The Monotremata (mon 6 trem' a ta; G., monos, 

 single, and trema, hole) are found in Australia, New Guinea, and Tas- 

 mania. They are mammals which lay eggs resembling the eggs of 

 reptiles and birds in being abundantly supplied with yolk and albumen 

 and covered by a shell. Monotremes also resemble those animals in 

 having a cloaca. In the case of the duckbills (Fig. 281) the egg is 

 deposited in a nest constructed at the end of a subterranean tunnel 

 in the bank of a river, the nest being above the water level and the 

 outer end of the tunnel opening under the water. The egg is not incu- 

 bated but it soon hatches and the young is fed from milk produced by 

 milk glands. However, there are no teats, the milk being passed into 

 two grooves on the ventral side of the mother's body. In nursing, the 

 mother lies upon her back and the young animal laps the milk from the 

 milk grooves. The Australian anteater. Echidna, possesses a temporary 

 pouch in which the eggs are incubated. 



441. Marsupials. — In the case of the Marsupialia (mar su pi a' li a; 

 G., marsypion, pouch) the egg has a thin membrane and only a little 



