CHARACTERS OF PROTOTHERIA. 567 



The brain is smooth, the cerebellum is not covered by 

 the cerebrum, there is a large anterior commissure and 

 but a small corpus callosum. 



The food-canal ends in a cloaca. 



The right auriculo-ventricular valve in Omithorhynchus 

 is partly muscular as in Birds, while in other Mammals it 

 is membranous, and worked by papillary muscles which do 

 not enter into the substance of the valve. The temperature 

 of the body is said to be about 25-28° C. 



The ureters open, not into the bladder, but into the 

 urinogenital canal. 



The testes remain abdominal. The left ovary is larger 

 than the . right, as in Birds. The vasa deferentia open 

 separately in the urinogenital canal. So in the female 

 do the oviducts, and these have no fringed abdominal 

 apertures nor distinct uterine region. The penis is attached 

 to the ventral wall of the cloaca, and its canal is not con- 

 tinuous with that of the vasa deferentia. 



The ova are large, have abundant yolk, and undergo 

 meroblastic segmentation. The Prototheria are oviparous. 



The duck-mole, duck-billed platypus, or water-mole, 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. It seems to collect small animals 

 in its cheek-pouches, chewing 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 "strong, flexible, white shell," through which the young animal 

 has to break its way. 



The full-grown duckmole measures from eighteen to twenty inches 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 skull. The eyes are 

 very small ; there is no external ear-flap or pinna ; the nostrils lie near 

 the end of the upper part of the bill. The tail is short and flat. 



Horny plates, two on each jaw above and below, serve as teeth in 

 the adult. The true teeth are calcified as in other mammals, but they 

 do not last very long, being soon worn away and shed. The cheeks 

 form capacious pouches. The digits are clawed and connected by a 

 web which is best developed on the fore-limbs. The spur borne on the 

 heel seems to be sometimes used as a weapon, and as it persists only in 

 the males, is likely useful in contests between rivals. 



