569. 328 [May 



VI 



The History of the Mammalia in Europe and 

 North America 



THE story of the development of the races of warm-blooded 

 quadrupeds, or Mammalia, in the northern hemisphere, during 

 the Tertiary period, is one of the most interesting and important 

 chapters in philosophical natural history. It is true that only the 

 barest outlines of the subject have hitherto been revealed ; but suffi- 

 cient is known to arrive at some general conclusions which are not 

 likely to be affected by future discoveries. 



Mesozoic 



The earliest known animals exhibiting any close approach to the 

 Mammalia in the structure of their skeleton date back to the close 

 of the Primary or Palaeozoic Era. They are five-toed quadrupeds, 

 with most features which now characterise cold-blooded animals of 

 the class of reptiles ; but in several respects their skeleton resembles 

 that of the lowest surviving mammals — the monotremes of the 

 Australian region — while their dentition is often differentiated into 

 cutting teeth in front (like incisors), grasping teeth at the corners 

 (like canines), and grinding or crushing teeth at the sides (like 

 molars). In allusion to the latter peculiarity they are usually 

 known as Anomodontia. During the Permian and Triassic periods 

 these animals flourished both in the northern and in the southern 

 hemisphere ; but before the dawn of the -Jurassic they seem to have 

 become extinct in all parts of the world which have hitherto been 

 geologically examined. It is probable, indeed, that in some isolated 

 region some of them passed into mammals about that time ; for in 

 the Jurassic rocks both of Britain and North America there are 

 occasional remains of small mammals as large as rats, and the most 

 plausible explanation of these is, that they were accidental escapes 

 from some other land with a more advanced fauna, just as are the 

 rats and mice of the present day in the comparatively antique realm 

 of Australia. Another group of diminutive mammals of the Jurassic 

 type occurs in the Upper Cretaceous (Laramie Formation) of North 

 America; while quite at the base of the Tertiary Formations, both 

 in North America and Europe, true placental mammals begin to 

 appear, and very soon become the dominant animals of the land. 



