The name given to these Monkeys conveys their habit of sleeping by day, waking up in the evening, and leading a very restless life during the greater part of the night. They are small animals, with a large round head, short face, and very large eyes; their fur is kept close; they have a tail of some length, but it can only curl around objects without holding on. The body is short, and greatly resembling that of the Squirrel Monkey in some points. They are distinguished as follows:--The two middle upper front teeth (incisors) are broad, and the lower ones project in a slanting direction; the canines are moderately long. The ears are partly hidden amongst the hair of the head, and the eyes are large. There is a curious condition of the upper arm-bone (humerus) of these Monkeys, which they have in common with the different kinds of Cebus, the Squirrel Monkeys, and the little Ouistitis about to be mentioned. It is, moreover, seen in the Carnivora, or the flesh-eating animals. The lower part of the bone, where it is jointed to the two bones of the fore-arm, at the elbow, has one of its projections there (the inner condyle) perforated by a hole. This gives passage to the main artery of the limb and the main nerve, and the use of it appears to be to prevent the contracting muscles of the arm pressing upon these important structures. They resemble some of the lower animals, especially one of the Lemuroida, of the genus Stenops, in the length of the loin back-bones; and, indeed, relatively this lumbar region is longer in them than in any other Monkey. The rib-bearing back-bones are more numerous than in other Monkeys, and there are either fourteen or fifteen of them, and, moreover, their spines are much prolonged forwards, as in carnivorous quadrupeds. THE THREE-STRIPED OWL MONKEY.[97]