or THE AUSTEALIAN MARSUPIALIA. 189 



Peramelcs and Dasi/urus are not homologous throughout, since in the former he 

 recognizes a vestigial germ of a morphological second incisor {di. 2). This element, 

 however, is prohably not a member of the same series as the functional teeth, hut is more 

 likely equivalent to the calcified germ described as di. 2 by Wilson and Hill, or, in other 

 words, the predecessor of the functional second (morphological third incisor), and also 

 homologous with the germ described as di. 3 in the upper jaw, which, according to 

 "Wilson and Hill, is large, but degenerates early in Perameles, and is present and calcified 

 in Dasi/nms. 



Finally, there is evidence of the presence of vestigial germs in the posterior incisor 

 region in the Dasyuridse and Peramelidoe. Dependorf describes, in his fourth and fifth 

 stages in Perameles, a vestigial germ (di. 5), presumably corresponding to the fourth 

 lower functional tooth of the Didelphyidge. This element is not mentioned by Wilson 

 and Hill for Perameles, but is described by Woodward for Thi/lacomys {Peragale), and 

 by both tlie latter writer and by Dependorf for Lasi/urus. A fifth upper vestigial germ 

 is also mentioned by Dependorf for Lasyxirus, although it is not mentioned by Woodward, 

 this writer having apparently concluded that the reduction took place in the middle 

 of the series. Dependorf states his inability to recognize either the upper or lower 

 intermediate teeth described by Woodward for Dasyurus. Further study of the whole 

 question from the standpoint of comparative embryology is greatly to be desired. 



Canines. — These teeth call for little consideration. They present exactly the same 

 insectivorous characters in the smaller Dasym'inae, and, in the case of the upper, in the 

 more primitive of the Phalangerinae [Acrobates and Distcechurus), as in the Didelphyidjp, 

 with the partial exception of Caluromys. 



Premolars. — In the general characters of the premolars the smaller Dasyurinaj and 

 the Didelphyidoe are inseparable. The chief special features relate to tlie proportions 

 of the posterior teeth. As already mentioned above, the Dasyurinae present successive 

 stages of reduction of the posterior premolars, beginning with a condition in which thev 

 are slightly or disproportionately larger than the median teeth, and ending in their 

 total obliteration. This reduction has also been mentioned for certain of the Phalan- 

 gerina? {Acrobates and Distcechurus), and for two members, Thylacomys layotis and 

 Chceropus castanotis, of the Peramelidse. Peratherium presents a prototypal condition, 

 the posterior premolars being apparently always larger than the median teeth *. In 

 the existing Didelphyidae we find indications of the same process of reduction seen in 

 the Dasyurinse, with the difference that it docs not proceed to the obliteration of the 

 teeth or even to the loss of their functional value. Of all the existing DidelphvidfB 

 Peramys makes the closest approach to Peratherium, the upper posterior premolar 

 being always larger, and the lower posterior tooth slightly larger than, or equal to, the 

 median. 



* This statement is based on five specimens in the British Museum collection, including Z'. ylwmrtrrfj (23S8), 

 P. arverneasiis (27700 and 27810), an undetermined species (27811), and P. exilia (27S00, type), and also on tho 

 description given by Gervais (1859), by whom two of the specimens have been figured. Cf. Lydekker (1887). 



26* 



