Of the skeleton, the eye-sockets, or orbits, are directed forward, and have complete bony margins, which, however, are not {9}closed in by bone behind (as in Monkeys), but freely communicating beneath the post-orbital process (except in _Tarsius_) with the temporal hollow behind. In the young of some species the orbit is more enclosed than it is in the adult: the orifice for the lachrymal duct of the eye is placed external to the margin of the orbit: the hollow for the olfactory lobes of the brain is always large. [Illustration: Fig. 3. Foot of _Chirogale trichotis_, Günther. (P. Z. S., 1875, p. 79.)] Having four kinds of teeth, and a set in succession to the milk-teeth, they are Heterodont and Diphyodont. The dental formula is I2/2, C1/1, P3/3, M3/3 = 36 (_vide_ anteà, p. 6), and the upper jaw has a toothless space in the centre (except in the Aye-Aye). Of the upper teeth, the _incisors_ are sometimes absent, but generally present; if unequal in size the inner one is the larger of the two. The canines are prominent; the pre-molars all have a _cingulum_, or girdle, round the base, more or less enlarged backwards into a process ("talon" or "heel"); the anterior pre-molar vertically long and canine-shaped; the median and posterior with three main points (tubercles or cusps) and one or two smaller ones on the crown, and having a bar or ridge uniting the front inner with the hind outer cusp. The anterior and median _molars_ have three or four main cusps, and one {10}or two smaller or subsidiary ones on the crown; the cingulum is well developed. The posterior molars have generally three cusps. In the lower jaw the _incisors_ are close-set and comb-like, remarkable for protruding in front, like the teeth of a Rat or a Rabbit. The _canines_ also protrude horizontally, and, being placed alongside of the incisors, are difficult to distinguish from the latter excepting that they are broader and thicker. [Illustration: Fig. 4. Skull of Lemuroid. From Blanford's "Mammalia of British India" (by permission of the author).] [Illustration: Fig. 5. Skull of Anthropoid. From Blanford's "Mammalia of British India" (by permission of the author).] Of the _pre-molars_ the anterior are canine-shaped, the median and posterior ones have three main, and one or two subordinate, cusps on the crowns. In both the upper and lower _molars_, cross-bridges stretch between the outer and inner front cusps as well as between the outer and inner hind cusps. {11}There is an oblique ridge between the hind outer and the front inner cusp, and another is often present between the front outer cusp and the anterior "heel," producing, as Huxley has pointed out, almost a double crescentic pattern, as in many lower Mammals. The posterior molar has four or five cusps.