2X4 



MAMMALIA— ORDER XII.—MONOTREMATA. 



only about half the length of the head and body, and more or less covered 

 with short, fine hairs, while it has but little or no prehensile power. In the 

 hind-foot the fifth toe is considerably shorter than the second, and the third 



and fourth digits are equal, and but little 

 longer than the latter. From the absence 

 of prehensile power in the tail, it has been 

 inferred that these tiny little- opossums 

 are mainly terrestrial in their habits. A 

 small opossum recently described from 

 Chiloe Island, off the coast of Central 

 Chili, is regarded as representing a 

 separate genus under the name of 

 Dromiciops. A third genus contains only 

 the water-opossum, or yapock [Chironecks 

 mininixis), ranging from Guatemala to the 

 south of Brazil, and distinguished by its 

 parti - coloured fur and aquatic habits. 

 Structurally it is characterised by the 

 presence of a projecting tubercle on the 

 inner side of the fore-foot, giving the appearance of a sixth digit; and 

 likewise by the webbing of the hind-toes as far as their terminal pads, 

 whereby the first digit is much less opposable than in other opossums. In 

 size the yapock may be compared to the common-opossum. 



Fig. U6. i-YAi ' 



. ectes minimus). 



ORDER XII. — MONOTREMATA. 



Egg-Layik"g Majimals. 



The third and last sub-class of mammals — variously known as the Proto- 

 theria, of Ornithodelphia, and containing only the single order Monotrenjata 

 — differs much more widely from the other two than do the latter from one 

 another, and thereby approximates to the lower vertebrates. These strange 

 mammals, which are confined to the Nutogteic realm, are especially 

 characterised by laying eggs, after the manner of birds and reptiles ; although 

 the young subsequently developed from suon egtjs are nourished by milk from 

 the mammary glands of the female parent. Instead, however, of opening 

 into teats, these milk-glands discharge their secretion U])on the porous skin 

 of the inside of a more or less developed temporary pouch by means of 

 numerous small apertures. After hatching, the head of the young, which 

 develops a special suctorial mouth, is thrust into the pouch, and the milk 

 forced into its mouth by the contraction of special muscles. Structurally, 

 the milk-glands of the Monotremes correspond to the ordinary sweat-glands 

 of other manunals, and not to their milk-glands. From the other two sub- 

 classes of mammals, Monotremes differ in that the hinder extremity of the 

 body possesses but a single orifice, from which are discharged both the 

 waste-products and the reproductive elements ; the two egg-passages, or 

 oviducts, opening into the extremity of this cloaca, as the single excretory 

 passage is termed. In these respects the egg-laying manmials resemble 

 birds and reptiles. Similar resemblances are presented by the portion of 



