THE AGE OF REPTILES 



called the Rhaetic beds because they were first found in 

 the old Roman province of Rhaetia, which occupied an 

 Alpine district between Bavaria and Lombardy. Here 

 they were thickest, 3000 feet of limestones and shales; 

 but they have since been found either thicker or thinner 

 everywhere in England, and in the United States, as well 

 as in other parts of Europe wherever we can find the 

 Lias lying on the Trias. They are especially interesting, 

 because they contain the teeth of the earliest known traces 

 of the highest division of the animal kingdom the 

 mammals. These early mammals belonged to the lowest 

 of all the mammalian tribes the Marsupials, or pouched 

 animals., now so common in Australia. The little banded 

 ant-eater of South America, which lives upon insects and 

 is about the size of a rat, is probably something like the 

 first mammal, the Microlestes, in habit and appearance. 



Let us now return, however, to the reptiles of the 

 Jurassic period. It is so called from the Jura Mountains 

 which occupy the north-west of Switzerland, separating 

 that country from France. They are composed of a 

 thick series of clays, shales, and limestones, to which, in 

 1829, the name Jurassic was given by the French geolo- 

 gist Brogniart. It was soon found, however, that the 

 lower rocks of this period were very different from the 

 upper. The lower rocks were very shaly and clayey with 

 thinnish layers of limestone. These were called Lias. The 

 name Lias is derived from "layers" pronounced broadly 

 by the Somerset quarrymen as "lyers" a very suitable 

 name for the lower beds of the Lias especially, since the 

 alternation of thin beds of limestone and of shale gives 



237 



