OCR Text |
Show 1877.] PROF. OWEN ON A NEW SPECIES OF STHENURUS. 355 logous, but here median, longitudinal ridge. In sum, the upper and long premolar in Dorcopsis, like that of the short premolar of Dendrolagus, is generically distinct in its modifications from that of the premolar in the three known species of Sthenurus. In the upper molars of Sthenurus the prebasal ridge is relatively larger than in Dorcopsis and Dendrolagus, the mid link is better developed ; there is an increase of breadth from the first (df) to the third (m 2) molar in Sthenurus not shown in Dorcopsis (comp. figs. c?4, « 3 in Plate XXXVII. with fig. 2, pl. vii. P. Z. S. 1875). The generic type of the lower premolar is still more marked in Dorcopsis (ib. plate vii. fig. 3) and Dendrolagus as compared with its homologue in Sthenurus (Plate XXXVIII. figs. 9, 10, 16, 17); this is manifested in the latter genus by the bilobed character of the outer surface, conspicuously shown in the unworn, unexcluded crown of the tooth in the type species of the genus (Sthenurus atlas)1. In Dorcopsis the outer surface, like the inner one, of the lower premolar retains the simple vertically grooved surface as in the Po-toroo type (Hypsiprymnus). The tendency to that modification of Macropodal structure is further shown by the retention of the canines, though minute, in the upper jaw, by the predominance in size of the foremost upper incisor, i 1, and the production of its cutting surface beyond or below the horizontal line of wear of the two following incisors (12, ^3). The canines are relatively larger in Dendrolagus, as in the Hypsiprymninse. I have not found an upper canine in a Sthenurus of any age. In the fossil of the young Sthenurus atlas (the subject of plate xxv. fig. 2, Phil. Trans. 1876), with the dentition less advanced than in the present evidence of Sthenurus minor, the maxillo-premaxillary suture shows no trace of such tooth or of any socket of a shed rudiment ; yet the foremost incisor was not fully in place, and the crown of the third incisor required for exposure the removal of the premaxillary alveolar wall. The character of the crown of i 3, so detected, enabled me to determine the detached homologous teeth of old individuals of Sthenurus atlas. In all the pattern is distinct from that of i 3 in either Dorcopsis or Dendrolagus. Subsequent acquisitions of premaxillaries and their teeth of the larger species, Sthenurus brehus, showed the same generic character (plate xxviii. figs. 1, 3, ii, ra, i3, Phil. Trans. 1876). The outer or labial surface of the crown of i 3 is equally bisected by a subvertical linear groove or enamel-fold; the fore-and-aft breadth of the unworn crown exceeds that which appears in the side view of ii (Plate XXXVIII. fig. 11) in Sthenurus atlas, and equals the total breadth of that tooth. The three incisors on each side work on the same horizontal level. In all these characters Sthenurus deviates from the Halmaturine, Dorcopsine, and Hypsiprymnine types, and approaches that of the great Kangaroos represented by Macropus proper, Osphranter, and Boriogale (Phil. Trans. 1874, plate xx. figs. 17, 18, 19). [ Mitchell's ' Three Expeditions into the Interior of Eastern Australia,' 8vo, 1838, plate xxix. fig. 1; Phil. Trans. 1874, pl. xxii. figs. 4, 5, 6, p 3, pl. xxiv. |