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 cerial. 



VII. — Mole {Talpa europoed). — 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 

 micsethmoid. The teeth are numerous and in many cases 

 they are forty-four in number, corresponding to the typical 

 eutherian dentition of |-t-|-|. 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 Insectivora illustrate in their 

 dentition a type usually regarded as of early origin, the 



