xin PHYLUM CHORDATA 567 



form in the class, having three cones or cusps in a longitudinal 

 row. In Dromatherium and its allies each molar has a single 

 main cusp with two smaller accessory cusps. There is no decisive 

 evidence as to the affinities of these primitive triconodont Mam- 

 mals, but they may be provisionally set down as allied to the 

 Prototheria. 



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FIG. 1159. Plagiaulax becklesi. Mandible with teeth. (After Owen.) 



Of the remainder of the Mesozoic Mammals some were probably 

 Prototheria, others Metatheria, while others again may have been 

 Insectivores. Most of them fall into two main groups. The type 

 of dentition presented by the members of one of these groups 

 (Fig. 1158) is more nearly allied to that of the Polyprotodont 

 Marsupials (p. 530) than to any other. In the other group (Multi- 

 tuberculata) (Fig. 1159) there is a superficial resemblance to the 

 Diprotodont Marsupials ; there is a single chisel-shaped incisor on 

 each side of the lower jaw, and one large, and sometimes one or 

 two smaller, on each side of the upper. A wide diastema separates 

 these from the pre-molars. The molars present longitudinal rows 

 of tubercles. In some cases the pre-molars have a pattern similar 

 to that exhibited by the molars, but in others they have a cutting 

 edge which may be serrated or obliquely grooved. An essential 

 difference between this dentition, and that of the existing Dipro- 

 todont Marsupials, in addition to the peculiar structure of the 

 molars, is that in the Multituberculata, when the upper incisors 

 are fewer than three, the large tooth which bites against the large 

 incisor of the lower jaw is the second and not the first. Moreover, 

 the fact that the vestigial molar teeth of Ornithorhynchus come 

 nearer in pattern to those of the Multituberculata than to those of 

 any other known group seems to indicate that the affinities of the 

 latter are rather with the Prototheria than with the Metatheria. 



Fossil remains of Mammals belonging to the Cretaceous age are 

 known only from certain limited beds in North America. But 

 in deposits of the succeeding Tertiary period there have been 

 found the remains of an extensive and varied Mammalian fauna. 

 The earlier Tertiary Mammals in many cases present features which 

 enable us without hesitation to refer them to one or other of the 

 existing orders : but when this is the case there is nearly always to 



