ORDERS OF MAMMALIA. 319 



CHAPTEE XXXI. 



OEDEES OF MAMMALIA. 



Order I. Monotremata. 



The Older of the Monotremes compiise.s only two very i-emr.rkable 

 genera of Mammuls — namely, Omithorhynchus (the Dvick- moles), 

 and Ecliidiia (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 Maninnilian tyjie. That they are 

 genuine Mammals is shown by their possession of hair, by the pre.s- 

 ence of mammary glands, Ijy the natm-e of their red libjud-corpuscles, 

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

 articulates with tlie 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 the}' pi'oduce 

 their young in the form of eggs. 



As regards their more important characters, the J/uJiut/rmata are 

 distinguished as follows : — 



The alimentary canal tei'mniates, as in Birds and Eeptiles, in a 

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

 hitestine, but also the ducts of tlie urinary' and reproductive organs. 

 The skull has the bones so amalgamated i]i the adult that the 

 majoi-ity of the sutuies aie obliterated, as occurs also in Birds. 

 There are no true teeth ; but in Ornithorhynchus the jaws carry 

 horny jilates (tig. i-HS, B and C), wdiich 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 comjilete ])Oiich developed on tlie abdomen of tlie females, 

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

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

 be regarded as formed bv a conversion into bone <jf the tendons of 



