CHAPTER VIII. 



THE TEETH OF MAMMALS. 



THE class Mammalia is divided into three groups : 



I. Ornitliodelpliia. 



Animals with a common genito-urinary chamber, and separate 

 coracoid bones ; no vagina ; no teats ; comprises a single order, 

 Monotremata, which contains only two genera, the Ornithorhynchus 

 and the Echidna. 



II. DidclpMa. 



Animals with a vagina, &c. ; of which the young are born in an 

 exceedingly early condition, probably without the formation of any 

 placenta, and are transferred to the nipple of the mother, where, 

 in almost all, they are protected by a fold of the abdominal integu- 

 ment, which forms the marsupium, or pouch ; comprises the single 

 order Mwsupialia, animals now most largely represented in Aus- 

 tralia and its zoological region ; some few exist also in America. 



The kangaroos, wombats, opossums, &c., are familiar examples 

 of Marsupials. 



III. MonodelpMa. 



Placental mammals : i.e., animals in which the foetus acquires a 

 connection with the parent through the medium of a vascular pla- 

 centa, by means of which it is nourished for a long time, and is 

 ultimately born in an advanced condition. 



The relations which the different orders of placental Mammalia 

 bear to one another are rather complex, and it is not possible to 

 place them satisfactorily in a consecutive series, because many of 

 the orders present affinities with, and are indeed linked by transi- 

 tional forms to, not one. but several other orders. Professor Flower 

 (Osteology, page 6,) has arranged them in the following tabular 

 manner, each order being placed near to those to which it presents 

 most resemblance. 



