288 



EGG-LAYING MAMMALS. 



districts, and subsisting almost exclusively on ants. They are generally found 

 in the mountains, and the three-toed species has been taken at an elevation of 

 between three and four thousand feet. Although it is definitely ascertained that 



they lay eggs, much less is known of their breeding- 

 habits than is the case with the duckbill ; according, 

 however, to native reports, the young, which are 

 probably two in number, are born during the Aus- 

 tralian winter, generally in the month of May. 

 Remains of a large extinct echidna have been obtained 

 from the superficial deposits of New South Wales. 



Allied Extixct Mammals. 



Certain forms from the Secondary and early 

 Tertiary rocks of Europe, Africa, and North America 

 are believed to belong to the Prototherian subclass, of 

 which they probably indicate a distinct order. Their 

 molar teeth have a distant resemblance to the teeth 

 of the duckbill, while the bones of the shoulder seem 

 to have comprised the two elements characterising 

 the Egg -laying Mammals. The peculiarity in the 



teeth of these mammals is that the molars are traversed by one or two longitudinal 



grooves, on either side of which are ridges carrying a number of small tubercles; 



and from this feature the name of Multituberculata has been proposed for the 



group. The number of ridges in the upper 



molars is always one more than in those 



of the, lower jaw. In some species, as in 



Tritylodon, represented in our hrst figure, 



the premolar teeth are similar to the molars ; 



but in others, as in our second figure, the 



molars are small, while the premolars are 



large and have sharp cutting edges. When 



unworn, such cutting premolar teeth gener- 

 ally have a series of oblique grooves on the 



sides, and as the incisor teeth (a) are large 



and often reduced to one pair, the jaw resembles that of the rat-kangaroos. The 



molar teeth, however, are different, and if these Secondary Mammals are really 



Prototherians, the character of their teeth indicates that they cannot be the 



ancestral types of the higher groups of the class. 



UNDER TART OF THE SKULL OF A 

 SOUTH AFRICAN SECONDARY MAM- 

 MAL, (ij uat. size). 



lower jaw OF plagiaulax (nat. size and 

 enlarged). — After Marsh. 



