MONOTEEMATA AND MAESUPIALIA. 



287 



The two living genera Ornitlwrhynchus and Echidna, which 

 alone comprise this order, are exclusively confined to the 

 Australian province, and possess some remarkable peculiarities 

 of structure. The most im- 

 portant of these, as regards the 

 skeleton, concern the condition 

 of the pectoral arch. Thus the 

 coracoids reach the top of the 

 sternum, with which they artic- 

 ulate ; while there is a large 

 T - shaped " intercla\dcle," or 

 " episternum " (fig. 597, i), 

 which supports the clavicles. 

 The condition of the pectoral 

 arch thus reminds one of that 

 which obtains in the Birds and 

 many of the Eeptiles. 



Palseontologically, the interest 

 of the Monotremes is chiefly 

 theoretical ; since the only fos- 

 sil remains referable to this 

 order that have hitherto been 



discovered, are those of a gigantic Echidna, recorded by 

 Mr Krefft as occurring in the Post - Tertiary deposits of 

 Australia. 



Oeder II. Maesupialia. • — The order Ilarsupialicc forms 

 with the Monotr&niata the division of the Non - placental 

 Mammals. With the single exception of the family Didel- 

 phidce, which is American, all the living Marsuincdia belong 

 to the Melanesian province — that is to say, they all belong 

 to Australia, Van Diemen's Land, Xew Guinea, and some of 

 the neighbouring islands. 



The following are the characters which distinguish the 

 order : — 



Tlie shull is composed of distinct cranial hones ivnited hy 

 sutures, and they all possess true teeth ; vjhilst the angle of the 

 lower jaiv is almost cdioays inflected. The pectoral arch has 

 the same form as in the higher Mammcds, and the coracoid no 

 longer reaches the anterior end of the sternum. All possess the 



Fig. 597. — sternum and adjacent parts 

 of the skeleton of a young Ornithorhyn- 

 chus (after Flower) : c, c, Clartcles ; i, 

 Interelavlele ; p, Prcestemuni ; 7ns, Meso- 

 stemum ; r, r. Vertebral ribs ; ir. Inter- 

 mediate ribs ; sr. Sternal ribs. 



