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 
atatime. 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 18 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 Proechidna 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 Echédua, more sparsely in Proechidua 
among the hairs. The snout is prolonged into a slender tube. There 
is a distinct pinna about an inch Jong. 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, 
Microlestes, Tritylodon. They are often classed with the Marsupials, 
Sub-Class METATHERIA, DIDELPHIA, or MARSUPIALIA 
With the exception of the American opossums, and 
a littleknown mouse-like animal (Cexolestes) from S. 
America, all the Marsupials now alive are natives of 
