OF THE AUSTBALIAN MARiSUPIALlA. ll^ 



ThYLACOMYIN-E. 



The antemolar teeth of Thylacomys call for no description, since they resemble in their 

 characters those of the Peramelinae and throw no further light on the evolution of the 

 family. 



The most important features relate to the characters of the upper molar patterns- 

 As will be seen from a comparison of PI. 5. fig. 11 with figs. 8, 9, 10, the upper 

 molars show a quadrate contour very like that seen in the more advanced forms of the 

 Peramelinai, but that while in the latter the quadrate shape is due to the development 

 of a liypocone, in Thijlacomys it is due to the displacement inwards of the metacone, 

 or, what amounts to the same thing, a shortening of the protocone. Tlie paiacone 

 retains its original characters and position. Of the external styles, only b and c are 

 represented. They are greatly enlarged and form with the protocone and metacone the 

 main cusps of a functionally quadrituberculate crown. The second tooth here figiired 

 will be seen to show a small hypocone wedged in betAveen the protocone and the meta- 

 cone. Tills element is absent in all of the other teeth of this sj)ecimen, including the 

 corresponding one of the opposite side. As to the extent of its occurrence in Thylacomys, 

 no further data have been available, except that it has not been figured or described by 

 Spencer (1896) in the new species T. minor. The specimen of T. leucurd here described 

 is rmfortunately the only example of this genus in the collection which shows the 

 luiworn molar patterns. 



The first molar of T. leucura resembles the second and third here figured. The fourth 

 is not visible in this specimen, but in adults of T. lagotis it appears to be of the same 

 reduced type as in the Peramelinae and Dasyuridse. 



The upper molar patterns of Thylacomys have apparently been derived from such a 

 primitive type as is presented by P. Doreyana or F. Boagainvillei, the rudimentary 

 hypocone having been in all proliability obliterated by encroachment of the metacone. 



The lower molars of T. lencura (PL 6. fig. 11^) are not modified away from the 

 perameline type, but present a much more advanced stage of development than is 

 found in any of the Peramelinse. The anterior lobe of each tooth no longer bears any 

 resemblance to a trigonid, the paraconid having completely disappeared. The; broad 

 ledge occupying the anterior border of the tooth represents the autero-extemal shelf, 

 a structure which is better developed in all of the Peramelidye than in the Dasyurinu'. 

 In the talonid the hypoconulid is quite vestigial, while the entoconid and hypoconid are 

 greatly enlarged and raised to the level of the anterior cusps. 



T'he lower molars of T. lagotis resemble those of T. leucura so far as can be judged 

 from worn specimens. In both species the hypsodont development is present to a 

 marked degree. 



The Cusp-homologies of Quadrate and Quadrituberculate Molars. 



The occurrence of two distinct lines of molar development in the Peramelidae raise.* 

 an interesting question as to the homologies of the cusps in more specialized forms. 



SECOND SERIES. — ZOOLOGY, VOL. IX. 17 



