In the skull the facial region is relatively small, and the cranial region relatively large. The external nostrils communicate with a cavity on the underlying bone; the pre-maxillary bones are deeply excavated in front, and the anterior perforations in the bony palate, behind the incisor teeth, are large. The lower jaw has its angle large, produced backwards, the line of union of its two halves being long, and its lateral movements very limited. In regard to their dentition, the number of the milk-teeth in the young individual is greater than that of the permanent set in the adult, the formula of the former being I2/2, C1/1, P2/3 [M3/3], while that of the latter is I2/2, C1/0, P2/2, M3/3, the lower canine and one lower pre-molar having disappeared. In the upper jaw the incisors are very small, the outer one standing behind the inner one, with a space between the former and the canine; the canines are long, curved behind, and set close up to the anterior pre-molar. The pre-molars are longer than they are broad, laterally compressed, and present to the outside one main triangular cusp with a small accessory cusp on each {92}side, the posterior tooth of the series having a hind inner cusp. The anterior and median molars are four-cusped, of which the outer and inner pairs are separated by a longitudinal groove; to the outside they have one supernumerary cusp on each main cusp, and one between them. The median molar is the largest tooth of the jaw, and the posterior is small, triangular and three-cusped. Of the lower jaw, the outer pair of the long, and almost horizontally protruding incisors, is larger than the inner pair, and is separated by a space from the anterior pre-molar. Of the elongate laterally compressed pre-molars, the anterior is the larger, and is vertically taller than its fellows, being slightly depressed forward and curved behind; the posterior pre-molar has one cusp. The molars have four cusps, of which the inner ones alternate with the outer cusps. The intestinal canal in the _Indrisinæ_ is very long, the cæcum, or blind diverticulum at the junction of its two portions, being extremely long and large, occupying, indeed, a great part of the abdominal cavity. The main arteries of the fore- and hind-limbs do not break up into a _rete mirabile_, or series of small parallel vessels, as in many other Lemuroids. In this group, while the sense of smell is very perfect, that of hearing is less acute than in the other Sub-families; and that of touch conspicuously blunt, both in the fingers and toes, which are chiefly climbing and not tactile and prehensile organs, as they are in the corresponding limbs of the Anthropoids. The female never produces more than one young at a birth. The convolutions of the brain are few, but they are more complicated than in many of the South American Monkeys. In very young individuals the cerebellum is more covered by the cerebrum than it is in the adult.