IV. VERTEBRATA: MAMMALIA, MONOTREMATA. 631 



Sub Class I. Monotremata (Ornithodelphia, Prototheria). 



A few mammals, confined to Australia and New Guinea, 

 divided among the genera Echidna, Proechidna, and Ornitho- 

 rhynchus, are the only living representatives of the group. They 

 are distinguished from all other mammals by laying eggs about 

 half an inch long, rich in yolk and with soft shells. These undergo 

 in the uterus a discoidal (meroblastic) segmentation and are then 

 incubated by Ornithorhynchus in a nest, by Echidna in a tempo- 

 rary pouch (marsupium) on the ventral surface of the body. On 

 hatching the young are nourished by the secretion of enormously 

 enlarged sweat glands, which form two large masses to the right 

 and left of the mid-ventral surface, and which must not be con- 

 founded with the milk glands (sebaceous) of other mammals. 

 Each opens on a special region of the ventral surface, which is 

 slit-like in OrnithorUynchus, a flattened pocket in the others. 



Other distinctions from other mammals, which are also points 

 of resemblance to reptiles and birds, are the strong development 

 of the episternum and the extension of the coracoid to the ster- 

 num (fig. 648), the termination of the ureters in the urogenital 

 sinus and not in the fundus of the bladder (fig. 653), the exist- 

 ence of a cloaca in both sexes, and the specifically bird-like char- 

 acter of the female sexual organs, in which the large left ovary is 

 alone functional, and uterus and 

 vagina are not differentiated. But 

 with all this it must not be forgot- 

 ten that the monotremes have the 

 hair, the skull, the urogenital sinus 

 of true mammals, and in the pres- 

 ence of marsupial bones (fig. 655, 

 Om) show a close relationship with 

 the marsupials. The upper end of 

 the hyoid is connected directly or by 

 a ligament with the cartilaginous 

 auditory opening, while a scarcely /* 



visible external ear occurs. The jaws 



, i i FIG. 655. Pelvis (left side) of Ornith. 



are toothless and enclosed in horny orhynchus paradox. (From Wie- 

 sheaths, yet in the young of Orni- S*T Ilium;' r^SEIfem'SS 

 thorhynchus there are in each jaw ^rsnpiai bone; p,os P ui 

 three pairs of multitubercular molars, which are later replaced 

 by four broad horny plates. 



