POUCHED MAMMALS. 191 



orders described above, no such division seems practicable in the case of the 

 Implacentals. Consequently the whole of them may be included in the single 

 order Marsupialia, which may be popularly known either as Marsupials or 

 Pouched Mammals. 



Both the technical and popular names of the group are derived from the 

 very general presence of a more or less fully-developed pouch on the 

 abdominal surface of the body of the female, into which the imperfectly- 

 developed young are transferred at birth, and where they remain until 

 sufficiently advanced to take care of themselves. This pouch is, however, 

 by no means universally present in the order, and cannot, therefore, be 

 taken as its distinctive character. The imperfect state of development of the 

 young at birth is, howevej, absolutely distinctive of all Marsupials ; and when 

 born the fcetuses resemble helpless and nearly motionless sacs of ilesh, which 

 are extremely small in proportion to the size of the parent, those of the large 

 kangaroos being no larger than a man's thumb. These helpless little abortions 

 are transferred by their parent immediately after birth to her teats, which 

 are always situated within the pouch whenever that'appendage is developed. 

 Of course such ill-developed creatures can do nothing in the way of sucking 

 by themselves, and Nature has accordingly provided a special arrangement 

 by means of which their wants can be supplied. This arrangement takes the 

 form of special muscles surrounding the milk glands of the female, which by a 

 spontaneous contraction comgresa the glands, and thus force the nourishing 

 fluid down the throats of the immovable, naked young. In order to prevent 

 the helpless young from being choked as they hang to the teats of the mother, 

 the upper part of the larynx, or superior extremity of the windpipe, is so elon- 

 gated as to project through the fauces, or hinder apertures of the mouth, and 

 thus reach the internal nostrils. In consequence of this arrangement a closed 

 passage is formed from the nostrils to the lungs, so that the foetus can 

 breathe while the milk flows down its throat. The young Marsupials thus 

 nourished hang on to the nipples of the mother until their bodies and limbs 

 are sufficiently developed to enable them to move about by themselves ; but, 

 as in the case of the kangaroos, they frequently resort to the pouch as a haven 

 of refuge from danger for a long time after they have made their appearance 

 in the outer world, and it is by no means uncommon for the pouch to be 

 tenanted by helpless fcetuses and half-grown young. Except in the thylacine, 

 where they are reduced to a rudimental condition, the pouch is supported by 

 a pair of marsupial or epipubic bones attached to the front edge of the lower 

 part of the pelvis ; these bones being also present in the male. 



In all the Pouched Mammals the brain is characterised by its propor- 

 tionately small size, and the slight development of the convolutions on its 

 surface. As already mentioned, the womb or uterus is double throughout 

 its entire length ; and in most cases the lower border of the angle, or hinder 

 extremity of the inferior margin of each branch of the lower jaw is more or 

 less strongly bent inwards. Although there is great variation, both numeri- 

 cally and structurally, in the dentition, yet there is one feature in regard to 

 the teeth characteristic of the order generally. This relates to the mode of 

 succession and replacement of the teeth. Whereas in the great majority of 

 terrestrial Placentals the whole or the greater number of the teeth 

 in advance of the true molars (which in that group are typically three in 

 number on each side of each jaw) are preceded by milk or baby-teeth, in the 

 Pouched Mammals only one tooth on each side of the jaws has such deciduous 

 predecessor, and even this is not invariably developed. Regarding the 



