i 2/3 c 1/1 pm 3/3 m 3/3, total 38.

The milk teeth are very slender and have sharp recurved cusps; they are quite unlike the permanent teeth. The permanent teeth are of two types. In the Insectivorous forms the molar teeth are cusped, and resemble those of Insectivora. In the blood-sucking Vampire bat Desmodus, the teeth are peculiarly modified; the canines and the single pair of upper incisors are much enlarged and exceedingly sharp, while all the other teeth are much reduced in size.

In the Frugivorous bats the molar teeth have nearly always smooth crowns. The dental formula in the chief genus Pteropus is i 2/2 c 1/1 pm 3/3 m 2/3, total 34.

The Primates have a diphyodont and heterodont dentition, generally of an omnivorous type, with cheek teeth adapted for grinding. The incisors are generally 2/2, and the molars, except in the Hapalidae, are 3/3. In the Lemurs the upper canines are large, and the lower incisors slender and directed almost horizontally forwards. The Aye Aye, Chiromys, has the following singular dentition: i 1/1 c 0/0 pm 1/0 m 3/3, total 18. The incisors much resemble those of rodents having persistent pulps, and enamel only on the anterior face.

In Man and in the Anthropoid and Old World Apes the dental formula is always i 2/2 c 1/1 pm 2/2 m 3/3, total 32.

In the Cebidae there is an extra premolar in each jaw bringing the number up to 36. In the Hapalidae, as in the Cebidae, there is a third premolar, but the molars are reduced to 2/2. Man is the only Primate that has the teeth arranged in a continuous series. In all the others there is a gap or diastema of larger or smaller size between the incisors and canines. In all except man also the canines are enlarged, especially in the males.

The Exoskeletal structures of mammals may be summarised in the following table:

I. Epidermal exoskeletal structures.

1.Hairs (a) ordinary hair,
(b) vibrissae and bristles,
(c) spines of hedgehog, porcupine, Echidna,
Centetes, Acanthomys.
2.Scales { of Manidae,
{ on tails of rats, beavers, &c.
3.Horns of Rhinoceros.
4.Horns of Bovine Ruminants.
5.Nails, claws, hoofs.
6.Spurs of male Ornithorhynchus and Echidna.
7.Horny beak and teeth of Ornithorhynchus.
8.Horny pads on jaws of Sirenians and Ruminants.
9.Baleen of whales.
10.Enamel of teeth.