18 CLASS MAMMALIA. 



of anomalous Didelphes are destitute of this organ. It is a 

 bone usually flattened, curved a little externally, enlarged 

 and thicker towards its base, and is attached, but not arti- 

 culated on the anterior edge of the os pubis, pretty near 

 the symphysis, so as almost to approach the opposite side. 

 It is more or less large according to the species. Its deve- 

 lopment seems to be in no degree proportioned to that of 

 the pouch. 



This bone is altogether peculiar, and no trace of it can be 

 found in the other Mammalia or in the Birds. It would 

 seem to have more analogy with bones placed in a simi- 

 lar situation in certain reptiles, such as for example the 

 Crocodiles. Here still is a considerable difference, for 

 these last are inserted into the muscles of the thigh, which 

 seems to render it probable that they are the bones of the 

 ilia, while in the Marsupiata, it is the fibres of the abdo- 

 minal muscles which are fixed there. It may, perhaps, be 

 safely asserted, that there is nothing precisely analogous to 

 this disposition to be found in the skeleton of any of the 

 vertebrata. 



Another, but a less important, though very constant 

 character is, the peculiar length of the symphysis of the 

 pubis ; in the formation the ossa ischii come in for a con- 

 siderable share. 



The peroneum has also more development than in any 

 other mammiferous animal. Its head is also constantly 

 articulated with the femur ; a disposition to be found in the 

 oviparous vertebrata, but not in the Mammalia. 



In the muscles, generally, fyc, there is nothing peculiar. 

 Of the pouch we have already spoken. As to the marsu- 

 pial bones, it is obvious that they can be acted on only by 

 the muscles of the abdomen, without, however, producing 

 any action on the pouch, which is wholly independent 

 of them. 



