o 
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 carl}'^ Mesozoic period, a few minute teeth, representing three 
small species, having been found in the lihietic 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 Phascolotheriumj^"\xi'Q(\. 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 
Palaeontological 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 jMarsupials, that is Didelphiau 
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 jieriod Mouodelphian 
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 fiom their Mesozoic ancestors. Thus the 
Eocene, the earliest of the Tertiary periods, has yielded remains of 
Bats, Insectivores, Carnivores, Kodentia, many Ungulates, Sirenia, 
and Cetacea. 
The Mammals of the ]\Iiocene, Pliocene, and Pleistocene periods, 
for which the Visitor is referred to the Geological Gufde, have 
increased in number and variety to the jircsent day; hut, at least 
for those that dwelt on the land, the maximum of size has long 
For the explanation of these terms see p. 90. 
