on the earth, and sufficiently distinct from each other to be regarded 

 by zoologists as species, has been estimated at about 3000, and 

 there are doubtless many, especially among those of smaller size, 

 still to be discovered. 



Mammals make their first appearance as far back as the Triassic 

 or early Mesozoic period, a few minute teeth, representing three 

 small species, having been found in the Rhsetic beds of Germany 

 and England. Later than these are the early Jurassic or Middle 

 Mesozoic Mammals, found at Stonesfield in Oxfordshire, where 

 several more or less complete lower jaws have been discovered, 

 such as those named Amphltherium and Phascolotherium, figured in 

 the Geological Guide, p. 77. In Upper Jurassic times also a very 

 large number of small mammals must have lived in this country, 

 as evidenced by remains found at Swanage, now exhibited in the 

 Palseontological Gallery. 



Of the exact affinities of these Mesozoic Mammals it is almost 

 impossible to form an idea ; but there can be little doubt that their 

 nearest living allies are the Marsupials, that is " Didelphian " 

 Mammals, in which the young are brought forth in an embryonic 

 condition, completing their development in a pouch formed of the 

 external integuments of the mother. To this day Marsupials show 

 the same division into two groups, according to their dentition, 

 which is observed in the ancient Swanage genera Plagiaulax, as 

 figured in the Geological Guide, representing the modern dipro- 

 todont *, and the others the polyprotodont * Marsupials. 



At the commencement of the Tertiary period te Monodelphian " 

 Mammals were already abundant, many of them resembling living 

 species a fact which shows how imperfect is our knowledge of the 

 intermediate time during which all these forms must have been 

 gradually developed from their Mesozoic ancestors. Thus the 

 Eocene, the earliest of the Tertiary periods, has yielded remains of 

 Bats, Insectivores, Carnivores, Rodentia, many Ungulates^ Sirenia, 

 and Cetacea. 



The Mammals of the Miocene, Pliocene, and Pleistocene periods, 



for which the Visitor is referred to the Geological Guide, have 



increased in number and variety to the present day; but, at least 



for those that dwelt on the land, the maximum of size has long 



For the explanation of these terms see p. 99. 



