582 



ZOOLOGY 



SECT. 



pattern similar to that exhibited by the molars, but in others they 

 have a cutting edge which may be serrated or obliquely grooved. 

 The fact that the vestigial molar teeth of Ornithorhynchus come 

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

 those of any other known group is looked upon as evidence that the 

 affinities of the latter are rather with the Prototheria than with the 

 Metatheria. In the American Cretaceous beds in which these 

 teeth are most abundant a number of limb-bones have also been 



found, some of 

 which show evi- 



dence 

 treme 



of Mono- 

 character- 



FIG. 1239. Phascolotherium bucklandi. Inner view of 

 right ramus of mandible. (After Owen.) 



istics. 



Fossil remains 

 of Mammals be 

 longing 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 

 existin'g orders ; but when this is the case there is nearly always to 

 be recognised an absence, or a less advanced development, of some 

 of the more salient characteristics ; in other words, the earlier 

 Tertiary Mammals, when 

 referable to existing 

 orders, are less highly 

 specialised than the 

 living representatives of 

 these orders. No less 

 significant is the fact 

 that these early Tertiary 

 representatives of exist- 

 ing orders had the cavity 

 of the brain-case nearly 

 always much smaller in proportion to the other dimensions than in 

 living forms. But many are not so readily referable to existing 

 orders, sometimes owing to their possessing marked special features 

 of their own, sometimes owing to their combining characteristic 

 features of two or more living orders. Through the series of 

 Tertiary and Post-tertiary formations it is possible to trace a gradual 

 development from the early generalised to the existing specialised 



FIG. 1240. Plagiaulax becklesi. 



teeth. (After Owen.) 



Mandible with 



