1266 CLASS MAMMALIA. 



family is represented solely by the genus Ornithorhynchus (fig. 1 1 3 1 ), 

 of which a single species inhabits the Australian rivers, in the banks 

 of which its burrows are constructed. According to the observa- 

 tions of Mr O. Thomas, it appears that there are usually two teeth 

 on either side of the upper and three in the lower jaw, which persist 

 till the animal is somewhat more than one-third grown. These teeth 

 have at first small but distinct roots, and present a distant resem- 

 blance to the true molars of some of the Multituberculata, like 

 Microlestes ; one of their longitudinal walls, or ridges, carrying a 

 number of minute cusps. It appears that the cornules grow up be- 

 neath and around these teeth, which are gradually worn away, and 

 finally shed like the milk-molars of other Mammals ; the hollows in 

 the cornules being the remnants of the original alveoli. 



Family Echidnid^. — The second family is characterised by the 

 convoluted cerebral hemispheres ; the production of the muzzle into 

 a long tube-like beak (fig. 11 30, a); the slenderness of the man- 

 dible ; the total absence of teeth ; and the presence of stout spines 

 mingled with the fur. Further, the feet are very strong, and adapted 

 for digging, and the centre of the acetabulum is imperfectly ossified. 

 There are two living species found in Australia and New Guinea, 

 both of which, at least for palaeontological purposes, may be included 

 in the genus Echidna, although the large E. Bruijnii, of New Guinea, 

 is frequently separated under the name of Proechidna. Remains 

 of a large species considerably exceeding the latter in size have been 

 obtained from the Pleistocene of New South Wales. 



Group Multituberculata. — In this place it will be convenient 

 to notice a group of very imperfectly known Mesozoic and early 

 Tertiary Mammals which were formerly regarded as Diprotodont 

 Marsupials allied to Thylacoleo, but which differ in several respects 

 from that group, and may perhaps eventually prove to be members 

 of the subclass Prototheria. Although these peculiar forms re- 

 semble the Diprotodonts in having a single pair of lower incisors 

 like those of Rodents, while many of them also approximate to 

 certain members of the same group in having a secant and grooved 

 fourth premolar ; yet they differ in that it is the second in place of 

 the first upper incisor which becomes enlarged and opposed to 

 the incisor of the lower jaw ; while when the fourth lower premolar 

 is secant the summit of its crown is extremely convex instead of 

 more or less concave. The true molars, as already mentioned, 

 appear to approximate in general structure to the deciduous cheek- 

 teeth of Ornithorhynchus, and are quite unlike those of any recent 

 Marsupials. Till, however, the structure of the pectoral girdle be 

 known the serial position of these forms cannot be definitely deter- 

 mined. And it may be observed, that the humerus of the limb 

 which may be referable to Tritylodon is unlike that of the Mono- 



