OCR Text |
Show 1900.] FOSSIL MARSUPIAL FROM TASMANIA. 781 meatus is directed outwards and a little forwards. The glenoid cavity is placed neither very high nor very low in regard to the base of the skull; it is higher than in the Dasyuridae, where it is more nearly on a level with the base of the skull, and somewhat lower than in the Phalangeridae, though it approximates more nearly to the position in the latter. The frontals show only a faint trace of a postfrontal process tending to bound posteriorly an orbital space ; their greatest width occurs in the cranial and not in the facial portion-a feature which is the reverse of that which is characteristic of marsupials, amongst whom the broadest part of the frontal is characteristically found in the facial part of the skull. Dorsally the frontal region is marked by a concavity of a V-shape, sharply outlined. This concavity is nothing like so wide nor so extensive in length as it is in Cuscus, Trichosurus, and Pseudochirus, from which it differs markedly in appearance and very much more resembles that of Dasyurus. Its posterior end, which practically corresponds in position with that of tbe least intertemporal width, is marked by the commencement of the sagittal crest. The lachrymals are of large size and extend upon the face to such an extent that, owing partly to the width of the nasal bone, they come into contact with the latter and so prevent the upward process of the maxilla from reaching the nasal. This feature is also met with in Phascolomys, while in some of the Phalangeridae also tbe lachrymal extends very nearly to the nasal. A. very characteristic feature of marsupials is seen in the extraorbital position of the lachrymal foramen, which is single and placed as in Sarcophilus. The nasals present two points of importance-first, the considerable expansion of their upper ends, the greatest width occurring at rather more than a quarter of the length from this point; and second, the forward extension of the bones beyond the premaxillae so that they overhang the nasal cavity. Their greatest length is 40'5 m m . and the greatest width 18-5 m m . The expansion which brings tbe nasals into contact with the lachrymals is indeed more marked thau in the Phalangeridae, Dasyurus, and certain Didel-pbyidae in which it is characteristically present. In the forward projection of the bones over the nasal cavity, tbe skull differs from that of the Dasyuridae and agrees with those of Phalanger, Trichosurus, Pseudochirus, and certain species of Didelphys ; iu fact the general structure and relationships of the nasal bones recall most strongly that which obtains in the latter. The lower part of the maxillary bone is broken away at the level of the infraorbital foramen, but there are two features of importance. First, the upward process, as already described, is completely shut off from contact with the frontal; and second, the infraorbital foramen is situated close to the suture between the maxilla and premaxilla, being considerably farther away from the orbit than in any existing marsupial. The premaxillae are of large side, approximating most in form |