Subclass I. PROTOTHERIA. 



The MONOTREMATA 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 females. 

 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. Mammae rudimentary. 



