248 MAMMALIA. 



Sub-Class 1. Prototheria. 



There are two chief difficulties in recognizing the remains 

 of the extinct Prototherian mammals. Firstly, all the suspected 

 known specimens are extremely fragmentary. Secondly, the 

 extinct forms already indicated by these fossils seem to be 

 highly specialized in one direction, while the few surviving 

 representatives of the sub-class are still more highly specialized 

 in another direction. Two little toothless burrowing animals 

 like the Ornithorhynchus and Echidna, can only afford a very 

 inadequate conception of the great group to which they belong; 

 while the isolated jaws, teeth, and limb-bones hitherto found 

 fossil are still less satisfactory for the purpose. However, 

 there is a remarkable resemblance between the early-shed teeth 

 of the immature Ornithorhynchus and the " multituberculate " 

 -(ftr " polybunous ") molars in certain jaws found in Mesozoic 

 and Lower Eocene strata. There is thus some justification for 

 placing these fossils in a special group or order, variously 

 named MULTITUBERCULATA or ALLOTHERIA, which may be 

 assigned at least provisionally to the Prototherian sub-class. 



ORDER 1. MULTITUBERCULATA. 



Even now it is impossible to determine how many of the 

 earliest remains of this character met with in Rhsetic and 

 Triassic rocks belong to the Anomodont reptiles. There is 

 not much doubt that Tritylodon, from the Karoo Formation 

 of South Africa, is referable to the latter category (p. 154); 

 and yet its tuberculated grinding teeth exhibit deeply cleft 

 roots. There is also some reason to suspect that other double- 

 rooted multituberculate teeth from the Rhajtic of Europe, 

 commonly claimed as mammalian, may be similarly interpreted. 

 The latter, however, have hitherto only been found isolated 

 and thus do not admit of satisfactory discussion. The known 

 specimens are quite minute, and the crown presents a median 

 'depression with a tuberculated or simply crenulated rim, like 

 the two hinder molars of Ptilodus (fig. 148 c). They are 

 named Microlestes, and two forms are recognized, M. antiquus, 

 from Wlirtemberg, and M. moorei, from Holwell, near Frorne, 



