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Show 28 DR. E. LONNBERG ON DIGESTIVE [Jan. 14, blance is the greater because they have a more horizontal direction than in the adult, in which latter they are also stouter, compressed, and only provided with an edge at the end. In the young the lower incisors, on account of their shape, work against the two median pairs of the upper incisors when the jaws are shut. In the adult they work only _ against the inside of the median pair of upper incisors when in a normal situation close to each other. The halves of the lower jaw are, however, movable, more so in the young than in the adult. This, together with the sideway movements of the lower jaw, explains also why the second pair of upper incisors are worn. The faculty of moving the mandibular halves so that the lower incisors may be separated from each other in the act of biting is, of course, very useful in many cases \ Thus, for instance, the animal is capable of securing a much larger piece of some soft fruit2 in one bite through this arrangement, and when occasionally preying upon animals or birds this faculty is also of importance. The mobility of the mandibular halves consequently serves here other purposes than in the Kangaroos and Pseudochirus. The upper canines are well developed in young and adult. The molars of the young Cuscus show four well-developed pyramidal cusps with radiating ridges, so that, as has been shown by O. Thomas, they resemble in some degree those of the Koala. The enamel of these cusps is, however, less developed in the Cuscus, so that they are in the adult animal soon worn down to such an extent that the crown becomes almost even, and only peripherally surrounded by enamel. The teeth are then not much adapted for any grinding action. The situation and different sloping of the posterior and anterior molars are similar to those described in 'Trichosurus. The action of the jaws must consequently be similar, although the enamel is rather less developed. To crush the pulp of fruits and similar matter the teeth are, however, sufficient. The hindmost premolar of both jaws lying just in front of the molar series is somewhat more strongly developed than in Trichosurus, pointed and reminding one a little of a canine. Those of the upper and lower jaw do not touch each other as in Trichosurus, but the mandibular premolar goes inside and in front of that of the maxillary. This development of the last premolar may have some connection with the alleged occasionally predatory habits of the animal. In Petaurus the median lower incisors are very long and slender. The median pair of upper incisors are longer than the others. They may thus, together with the lower incisors, form a pair of pincers. It is also probable that the mandibular incisors themselves may, because both halves of the lower jaw are quite movable, act as a suitable implement for pinching and scratching 1 The mobility of the mandibular halves of the Rodents and its causes have been extensively discussed by Tullberg in his work quoted above, p. 345 and following 2 Tullberg has stated that Squirrels feeding on mushrooms separate their incisors (I. c). |