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Show 1902.] ADAPTATIONS IN DIPROTODONT MARSUPIALS. 29 small insects from their refuge in flowers, in cracks in the bark, and similar places where these slender incisors may conveniently be inserted. When securing larger insects this can, of course, be more easily done with the incisors separated so that they act as a fork, than if they lie close together and form only one point. The molars have four moderately developed bluntly pointed cusps. The lateral row of cusps of the mandibular molars fits in between both rows of cusps of the upper molars. On the whole the dentition may be said to approach the insectivorous type. The molars can certainly not be used for the grinding of any hard vegetable matter, and the incisors are too weak to gnaw. In Acrobates the development has gone still further in the same direction. The median lower incisors are long and slender, although, if compared with the skull itself, not so long as in Petaurus, which has a shorter, less pointed snout. They may certainly serve as pincers and the mandibular halves are quite movable. The premolars of Acrobates are much better developed, longer, and more pointed than those of Petaurus. When the jaws shut, the premolars of the upper and lower jaws meet, and the latter slide up in front of the former. These teeth may thus help in catching and holding the prey, which is not the case in Petaurus. In the latter the premolars and second incisors of the lower jaw are small and functionless. This is because, in consequence of the length of the median incisors and the corresponding shortness of the jaw itself, there is formed a considerable opening between the upper and the lower jaw corresponding to the canine and premolar region of the maxillary. The maxillary teeth thus cannot meet the mandibular teeth, which do not even lie opposite to them. The molars of Acrobates are similar to those of Petaurus, but their cusps are sharper. It may be in consequence of the arrangement of the premolars and their use that Acrobates has been able to reduce its number of molars to 3/3 when Petaurus has 4/4. In none of the Phalangerids which have the rami of the lower jaw movable, as described above, have I been able to detect in m y material any trace of such a transverse muscle as that which is found in the Kangaroos at the base of the mandibular incisors, and which has the function of approximating the inner edges of these teeth. In the Kangaroos it is said by Leche1 that the mandibular incisors are separated from each other by the combined action of the muscidi biventer, mylohyoideus, and genio-hyoideus. In his great work on the Rodents already quoted, Tullberg states that m. masseter serves to break or bend the lower margin of the mandible outwards, and that in such a case the incisors become pressed close to each other. On the other hand, the m. transversus mandibulce, when contracting, approaches the lower margins of the mandibular rami towards the median line, 1 Bronn : Kl. u. Ordn. d. Thierr., Saiigethiere, vi. 5. 1. p. 681. |
