86 THE LESSON OF EVOLUTION 



existed in the Triassic period ; but this appears to be 

 doubtful, as Professor H. G. Seeley has shown that 

 Tritylodon and others are reptiles (Anomodontia), as is 

 proved by the existence of pre-frontal and post-frontal 

 bones in the skull, a small quadrate bone, and some- 

 times a composite mandible. On the other hand, they 

 had the mammalian characters of two occipital condyles, 

 and complicated molar teeth with divided roots. 1 



In Dromatlurium the roots of the teeth are imper- 

 fectly divided, and it may be put down as reptilian 

 without much hesitation; while the molar teeth, 

 known under the name of Microlestes, resemble those 

 of Plagiaulax, and may be mammalian. 



In the Jurassic, however, we have undoubted 

 mammals. The Plagiaulacidse have only two long 

 incisors in the lower jaw, separated by an interval 

 from the pre-molars, which are large and obliquely 

 grooved ; the true molars being small and tuberculate. 

 The family is also represented in the upper Cretaceous 

 of North America, and in the Cainozoic of Patagonia. 

 This family has been placed in the sub-class Proto- 

 theria on account of the resemblance of the molars to 

 the deciduous teeth of Ornithorhynchus ; but it forms a 

 special order, called the Multituberculata, the teeth of 

 which are difficult to distinguish from those of the 

 Theriodont reptiles. The sub-class, Metatheria, or 

 Marsupials, are represented by the families Phascolo- 

 theridse and Amphitheridse. In the former the lower 

 molars have three main cusps, and some accessories, 

 all in a line ; while, in the latter, the lower molars 

 are trituberculate in the anterior portion, and with 

 one tubercule in the posterior portion. Professor H. 

 1 Trans. Royal Society) Series B., vol. clxxxv. p. 1019. 



