METATHERIA, DIDELPHIA, OR MARSUPIALIA. 743 



many ways the copulatory organ of certain Reptiles and 

 Birds. 



The ova are large, with abundant yolk, and undergo 

 meroblastic segmentation. The Prototheria are oviparous. 



The duckmole, or duck-billed platypus, lives beside lakes and rivers. 

 It swims by means of its fore-limbs, which are webbed as well as 

 clawed ; it grubs for aquatic insects, crustaceans, and worms, in the 

 mud at the bottom of the water or among the floating weeds. It 

 collects small animals in its cheek pouches, and chews them at leisure 

 with its eight horny jaw-plates. It makes long burrows in the banks, 

 often with two openings, one above, one under the water. The animal 

 is shy, and dives swiftly. when alarmed. When about to sleep, it rolls 

 itself into a ball. In the recesses of the burrows the eggs are laid, two 

 at a time. The egg measures about three-quarters of an inch in length, 

 and is enclosed in a flexible white shell, through which the young 

 animal has to break its way. 



The full-grown duckmole measures from 1 8 to 20 in. in length ; the 

 male -slightly exceeds his mate. The fur is short and soft, dark brown 

 above, lighter beneath. The jaws are flattened like the bill of a duck, 

 and covered with naked skin, which forms a soft, sensitive collar around 

 the region where the bill joins the rest of the skull. The eyes are very 

 small. There is a well-developed but inconspicuous pinna ; the nostrils 

 lie near the end of the upper part of the bill. The tail is flat. 



True teeth, three on each jaw above and below, are calcified, last for 

 about a year, and are then lost, being replaced by horny plates, two on 

 each jaw, above and below. The spur borne on the heel seems to be 

 sometimes used as a weapon, and as it persists only in the males, is 

 perhaps useful in contests between rivals. 



Echidna and Froechidna live in rocky regions, are mainly nocturnal 

 in habit, and burrow rapidly, legs foremost. They feed on ants, which 

 are caught on the rapidly mobile, slender, viscid tongue. No traces of 

 teeth have as yet been seen. 



Strong spines occur thickly in Echidna^ more sparsely in Proechidna 

 among the hairs. The snout is prolonged into a slender tube. There 

 is a distinct pinna about an inch long. The limbs bear five toes, two 

 of which in Proechidna are often without claws and somewhat rudi- 

 mentary. In Echidna the eggs seem to be hatched in a temporarily 

 developed pouch, which is apparently comparable to a much-expanded 

 mamma of the type seen in the cow. 



The Allotheria or Multituberculata include small extinct Mammals 

 (from Triassic to Eocene) with multituberculate molars, e.g. Plagiaulax t 

 Microlestesy Tritylodon. They are often classed with the Marsupials. 



Sub-Class METATHERIA, DIDELPHIA, or MARSUPIALIA 



With the exception of the American opossums, and 

 a little-known mouse-like animal (Canolestcs) from S. 

 America, all the Marsupials now alive are natives of 



