LATEE LIFE ON THE EARTH 53 



thyornis must have been a powerful flyer, apparently 

 feeding upon fish. In the upper Cretaceous of 

 North America remains of other birds have been 

 found, some of which may belong to the living group 

 of carinate birds. 



For a long time it was thought that mammals 

 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 sometimes a composite mandible. On the other 

 hand, they had the mammalian characters of two 

 occipital condyles, and complicated molar teeth with 

 divided roots. 12 



In Droma'therium the roots of the teeth are im- 

 perfectly divided, and it may be put down as rep- 

 tilian without much hesitation ; while the molar 

 teeth, known under the name of Microlestes, re- 

 semble those of Plagiaulax, and may be mammalian. 



In the Jurassic, however, we have undoubted 

 mammals. The Plagiaulacidae 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 tuber- 

 culate. 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 Prototheria on account of the resemblance 

 of the molars to the deciduous teeth of Ornithorh- 

 ynchus ; but it forms a special order, called the 



"Trans. Royal Society, Series B., vol. clxxxv., p. 1019. 



