MAMMALIA. 537 
ARBOREAL TYPE. 
These all dwell typically in trees. As examples we may cite—the 
marten, polecat, lemur, monkey, pangolin, opossum, tree-shrew, squirrel, 
tree hyrax, tree anteater, phalangers, sloths. 
1. The “incidental” group: marten, polecat, pangolin, squirrels. 
In these types a terrestrial life in forests is indulged in and the animals 
can walk with ease on the earth, but resort to the trees for food or 
shelter. The limbs usually commence to show a ‘‘climbing” form, 
the claws are sharp and the animal ‘hangs on” to the tree by this 
means. 
2. The ‘‘transition” group: monkeys, lemurs, opossums. Here 
the “tree” and ‘‘ground” habit are both indulged in, but the arboreal 
adaptations are marked. The first digit becomes opposable to the other 
four to form a climbing ‘‘grip.”” The limb-bones are all retained and 
partake of the ‘‘climbing” characters. 
3. The true arboreal type: sloth, tree anteater. In these the 
arboreal habit is predominant. The claws are permanently curved for 
hanging to the boughs and the number of digits tends to be reduced to 
three or two. Retia mirabilia are usually present to allow of free 
circulation in the vertically placed limbs. Both insectivorous and 
herbivorous diets are found. 
Like the-cursorial type, the arboreal is evidently derived from the 
primitive terrestrial and its incidental group has given rise to the eerial. 
VIIL—Mo te (Zalpa europea).—FossoriaL TYPE. 
The mole is the commonest and best known of the 
true fossorial or burrowing types and its anatomy is an 
object-lesson in adaptation. Externally we note the elon- 
gated cylindrical body, clothed in fine short fur which will 
lie with equal facility in either direction. There are no 
external ears, and the eyes are extremely minute, lying deep 
in the fur. The snout is pointed and the tail is small and 
stump-like. The skull is long and tapers to the front end, 
which is strengthened by the forward projection of the 
mesethmoid. The teeth are numerous and in many cases 
they are forty-four in number, corresponding to the typical 
eutherian dentition of 34-43. The mole belongs to the 
Insectivora, an order the members of which typically prey 
upon small invertebrate animals, such as insects and worms. 
This is naturally a more primitive mammalian diet than 
mammalian flesh or even grass or fruits, so it is not 
surprising to find that the /zsectivora illustrate in their 
dentition a type usually regarded as of early origin, the 
