MARSUPIALIA. 407 



lieved to be ossifications of the internal tendon of the external 

 oblique muscle of the abdomen. The corpus callosum is very 

 small, and has been asserted to be altogether wanting. There 

 are no external ears. The mammary glands have no nipples, 

 and their ducts open either into a kind of integumentary pouch 

 (Echidna^ or simply on a flat surface (Ornithorhynchus\ The 

 young are said to be destitute of a placenta, or, in other words, 

 no vascular connection is established between the foetus and 

 the mother. The feet have five toes each, armed with claws, 

 and the males carry perforated spurs on the back of the tarsus 

 (attached to a supplementary tarsal bone). 



As regards their geological history, the Monotremes are not 

 known to be represented at all in past time ; and this need not 

 excite any surprise, seeing that the order is represented at the 

 present day by no more than two genera, both confined to a 

 single geographical region. Upon theoretical grounds, how- 

 ever, it may be expected that we shall ultimately discover that 

 the antiquity of the order Monotremata is extremely high. 



ORDER II. MARSUPIALIA. The order Marsiipialia forms 

 with the Monotremata the division of the Non-placental Mam- 

 mals. With the single exception of the genus Didelphys, which 

 is American, all the Marsupialia belong to the Melanesian pro- 

 vince ; that is to say, they all belong to Australia, Van Diemen's 

 Land, New Guinea, and some of the neighbouring islands. 



The following are the characters which distinguish the 

 order : 



The skull is composed of distinct cranial bones united by 

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

 the lower jaw is almost always inflected. The pectoral arch 

 has the same form as in the higher Mammals, and the coracoid 

 no longer reaches the anterior end of the sternum. All pos- 

 sess the so-called "marsupial bones" (fig. 336), attached to 

 the brim of the pelvis. The corpus callosum is very small, 

 and has been asserted to be absent. The young Marsupials 

 are born in a very imperfect condition, of very small size, and 

 at a stage when their development has proceeded to a very 

 limited degree only. It is believed that there is no placenta 

 or vascular communication between the mother and foetus, 

 parturition taking place before any necessity arises for such an 

 arrangement. As the young are born in such an imperfect 

 state of development, special arrangements are required to 

 secure their existence. When born, they are therefore, in the 

 great majority of cases, transferred by the mother to a peculiar 

 pouch formed by a folding of the integument of the abdomen. 

 This pouch is known as the " marsupium," and gives the name 



