GENERAL CHARACTERISTICS. 



193 



abdomen, and either surrounded by simple 

 folds of the skin, or contained in an actual 

 pouch. This pouch has only a single open- 

 ing, usually in front, but sometimes, as in 

 the pouched badgers ( Peramelida), behind. 

 It is nothing but a fold of the skin, and has 

 no connection whatever with the abdominal 

 cavity. In the ordinary position of these 

 animals, on their four feet, the marsupial 

 bones protect the pouch from the pressure 

 of the abdominal viscera, imparting as they 

 do greater solidity to the walls of the ab- 

 domen. According, to the observations made 

 in zoological gardens the mother takes the 

 young one by the mouth immediately after 

 birth, and attaches it to one of the teats in 

 the pouch ; the young one then remains hang- 

 ing there for a longer or a shorter period; 

 and even afterwards, when it is able to move 

 about freely, it frequently seeks refuge in the 

 pouch, which, however, it ultimately leaves 

 for ever. 



This peculiar mode of reproduction natu- 

 rally involves a considerable number of modi- 

 fications in the structure of the sexual organs ; 

 but on these we do not enter at present. 

 They are associated with other characteristic 

 features in the general organization, with the 

 aid of which it is always easy to distinguish 

 a marsupial, notwithstanding the considerable 

 divergences existing among themselves. 



We must lay great stress on the fact that 

 the marsupials do not form an order in the 

 sense in which the word is used with re- 

 ference to the other (the placental) mammals, 

 but rather a large group, composed of a 

 number of divergent types, each of which 

 forms an order by itself. In this group we 

 find carnivores, rodents, insect-eaters, and 

 herbivores, all well characterized, and this 

 diversity of types can be understood only 

 when we regard the marsupials of the present 

 day as the relics of an old order of things, 

 in which the entire class of the mammals was 

 represented solely by marsupials over the 

 whole earth. As we are taught by the facts 



Vol. II. 



of palaeontology, the former marsupials were 

 the stocks from which, on the one hand, the 

 placental mammals were evolved; while, on 

 the other hand, their less modified descen- 

 dants form the marsupials of the present day. 

 We need not be astonished, therefore, when 

 we observe that these descendants have pre- 

 served a number of primitive characters, and 

 that they occupy a very subordinate position 

 with respect to the other orders of mammals. 



The brain and the part of the skull inclos- 

 ing it are unusually small in relation to the 

 bones of the face and the jaws. The brain, 

 which is itself small, exhibits positive marks 

 inferring a low stage of development. The 

 hemispheres are smooth and never cover the 

 cerebellum. In most cases the corpora quad- 

 rigemina also remain uncovered; only in the 

 large species are there to be seen a few 

 faintly-marked convolutions on the hemi- 

 spheres. The corpus callosum, which is so 

 highly developed in the placental mammals, 

 consists in this group only of a. few unim- 

 portant fibres. The bones of the skull are 

 not fused. The lower jaw always shows an 

 inflection of the posterior angle inwards. 

 This last characteristic feature does not be- 

 long exclusively to the marsupials, for I have 

 also observed it in certain American rodents 

 — in the ursons of North America (Erethizon) 

 and the Mexican souslik {Spermophilus mexi- 

 canus). As a peculiarity of the skeleton we 

 may mention the marsupial bones. These 

 are two bones, mostly cylindrical in shape, 

 united to form a sort of fork, which is at- 

 tached to the pubic symphysis. 



The dentition exhibits considerable modi- 

 fications. In comparison to the placental 

 mammals the number of the teeth is in 

 general very high. In the carnivorous and 

 insectivorous marsupials we often find an 

 extraordinary number of incisors and cheek- 

 teeth; in the former the canines are always 

 developed, and the number of carnassial teeth 

 may be as many as three. But although in 

 the groups mentioned we find three or four 



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