OCR Text |
Show 1901.] LEMCR MONGOZ AND L. RUBRIVENTER. 267 Finally, the palatal sinus covers completely the orbital maxillary plate also, and constitutes for itself the whole inflated bottom of the orbit (PI. XXII. fig. 4). As has already been mentioned, the sinus expands also on the basis cranii; the most posterior part of tbe palatal, the processus pgramidalis, becomes inflated, and the choanse are considerably narrowed by the swollen pars perpendicularis (PI. XXII. fig. 7, pp). In very old specimens a partial absorption of the roof of this sinus takes place (PI. XXII. fig. 5). I have found this very curious palatal sinus in all the specimens of Lemur rubriventer examined, eight in the Zoological Department of the Natural History Museum, seventeen in the Leyden Museum, aud six specimens forming the remainder of m y collections made in Madagascar. It is one of the characteristics of pneumatic cavities in Mammalia, that they often vary greatly within the same genus, so as to offer excellent specific characters for systematic purposes, when, as is the case in the present and in the before described species (Lemur mongoz), the change in the skull is such that it becomes apparent without any dissection being necessary. The skull of Lemur rubriventer can at once be recognized by the unique feature of its bulla-shaped palatal sinus. As to its particular function, it certainly has not the one to supply space for the teeth. Conversely, a section of the maxillary sinus in this and other species shows that the latter apparently has amongst its functions to favour or protect the development of tbe two posterior premolars ; and in the L. mongoz before described, the peculiar posterior sinus affords a similar protection to the two anterior true molars. But the palatal sinus of L. rubriventer certainly interferes with the development of the last true molar, for its floor grows over the corresponding part of the maxilla at the very time when the tooth is in the state of germ. The fact that L. rubriventer is the one species of the genus which has the smallest and most reduced m. 3 (see PI. XXII, fig. 7) is a confirmation of the above. The distribution of these pneumatic cavities over the whole of the family (Lemuridae), and their various degree of development in different groups, gives a clue as to their principal function when they are greatly developed. In those Lemurs which are characterized by their sluggish movements (Loris, Nycticebus, Perodicticus), the pneumatic cavities are small. In the nearly related Galagos, known for their agility, the sinuses around the cavum nasale are equally reduced; but the pneumatic cavities of the mastoid region evidently act as a compensation. In Malagasy Lemurs inflations of the mastoid region are an exceptional occurrence and, when present, never greatly developed, except in " Chirogaleus trichotis." The maximum of development of aerial siuuses, almost exclusively of those starting from the cavum nasale, occurs in the larger forms of Malagasy Lemurs, all of them excellent climbers and acrobats. |