OKDERS OF MAMMALIA. 319 



CHAPTEE XXXI. 



OEDEES or MAMMALIA. 



Order I. Monotremata. 



The order of the Monotremes comprises only two very remarkable 

 genera of Mammals — namely, Ornitliorhynchus (the Duck-moles), 

 and Echidna (the Spiny Ant-eaters), which are characterised by the 

 singular combination in their structure of Eeptilian or Avian pecu- 

 liarities with features of a purely Mammalian type. That they are 

 genuine Mammals is shown by their possession of hair, by the pres- 

 ence of mammary glands, by the nature of their red blood-corpuscJes, 

 by the structure of the lower jaw, and by the fact that the skull 

 articulates with the vertebral column by two occipital condyles. On 

 the other hand, they resemble the Eeptiles or Birds, or both, as 

 regards the termination of the intestine in a cloaca, the structure of 

 the pectoral arch, and the want of true teeth, and in the still more 

 important fact — but lately clearly established — that they produce 

 their young in the form of eggs. 



As regards their more important chai-acters, the Monotremata are 

 distinguished as follows : — 



The alimentary canal terminates, as in Birds and Eeptiles, in a 

 dilated " cloaca," which not only receives the termination of the 

 intestine, but also the ducts of the urinary and reproductive organs. 

 The skull has the bones so amalgamated in the adult that the 

 majority of the sutures are obliterated, as occurs also in Birds. 

 There are no true teetli ; but in Ornithorkijnchus the jaws carry 

 horny plates (fig. 226, B and C), which act as teeth. The pectoral 

 arch, which supports the fore - limbs, resembles that of Birds in 

 several respects, but especially in the fact that the coracoid bones 

 are distinct, and are not amalgamated with the shoulder-blade. 

 There is no complete pouch developed on the abdomen of the females, 

 but there are the so-called " marsupial bones." These are two small 

 bones which arise from the front of the pelvis. They are really to 

 be regarded as formed by a conversion into bone of the tendons of 



