Subclass I—PROTOTHERIA. 
The MonoTrREMATA or ORNITHODELPHIA, as the Prototherian 
Mammals have been variously, and more or less misleadingly 
termed, comprise the lowest, and in point of time the most ancient, 
types of mammalian life. Two families only, consisting of three 
genera, exist at the present time, the extent of their range being 
confined to Australia and Tasmania ( Ornithorhynchus and 
Echidna) and New Guinea (Proechidna and Echidna). They are 
oviparous, and the young are nourished after emerging from the 
egg by milk expressed from the mammary glands into a temporary 
pouch ; these glands are not provided with nipples, the milk being 
forced out through numerous apertures in the spongy skin. The 
method employed in hatching out the eggs differs materially in the 
two families, for while the Platypus forms a nest in its burrow on 
which to deposit its eggs, and hatches them out by the warmth 
of its body in the same manner as birds do,—the pouch being at 
no time sufficiently developed to enable the animal to retain the 
eggs therein—the Echidna carries its eggs about with it and 
hatches them out in its more highly specialized pouch. The 
males are provided with a perforated spur on the inside of the 
heel, which is connected by a duct with a postfemoral gland, 
and the functions of which are not as yet clearly understood; the 
assumption that it is a poison-organ is not however borne out by 
actual evidence, while on the contrary it is difficult to imagine of 
what possible value such an organ could be to animals so consti- 
tuted ; it is, however, possible that it may develop such a function 
in a greater or less degree during the breeding season, and be used 
as a weapon of defence in contesting for the favor of the femaies. 
The Monotremes possess an additional clavicular bone, in the 
shape of a large T-shaped interclavicle, which bone is not 
represented in either of the other mammalian Subclasses ; the 
coracoid bone is complete and articulates with the sternum. 
Epipubic bones are present. True functional teeth are absent in 
the adults. 
Order I.-—MONOTREMATA. 
Muzzle produced into a beak, which is either flattened or 
cylindrical. External ear-opening without a conch, Limbs sub- 
equal in length, short and powerful. Tail either short and 
broad or rudimentary. Mamme rudimentary. 
A 
