388 DE. B. A. BENSLET ON THE EVOLUTION 



is placed behind that of the first, while that of the third is placed externally to that 

 of the second. 



In the incisor formula the Peramelidse are the most primitive of all the Australian 



forms, the number being ^ . The Dasjuridee come next with the uniform formula 

 of ^. The Didelphyidse present the prototypal condition of j . The mutual homologies 

 of the teeth in the three families are not at all clear. Thomas (1888), Winge (1895), and 

 Dependorf (1898) have estimated the original marsupial formula as ^ , but there is no 

 agreement as to how reduction from this type has been effected in the different families. 

 The various views may be tabulated as follows : — 



Thomas (1S8S). Winge(i895). 



-n-J 1 1, -A 12345 12345 



Didelphyidae TT3T6 0~2^45 



12 345 1 2345 



i:'erameliaie 12300 02340 



• L-Al? ]_2 3 4 



Dasyundse 12XOO 02340 



The evidence on this point is admittedly unsatisfactory, but it is probable that the 

 plan of reduction suggested by Winge is the correct one. In the first place, there is 

 evidence to show that the anterior lower incisor has disappeared throughout. Winge 

 has pointed out that the median lower incisors are in relation with the second upper 

 teeth, with which they are accordingly homologous. There are no lower teeth corre- 

 sponding to the median upper ones, and it may thus be supposed that the development 

 of a procumbent caniniform condition in them may have disturbed the relations of their 

 ori"-inal homologues in the lower jaw, leading to their reduction. Furthermore, both 

 Woodward and Dependorf have described vestigial germs situate in front of what are 

 later on the first functional lower teeth in embryos of Dasyurus. Wilson and Hill 

 mention a possibly equivalent germ in JPerameles, although this is not mentioned by 

 Dependorf for that genus. 



In the second place, there are reasons for believing the three lower incisors of the 

 Dasvurida3 and Peramelidse to be homologous with one another and with the first three 

 lower incisors of the Didelphyidse. Wilson and Hill have called attention to the fact 

 that in embryos of Perameles the germs of the lower functional teeth occupy a peculiar 

 relation to one another, the second being situate in a line posterior to the first, while 

 the third, in consequence of a sharp bending outwards of the dental lamina, is situate 

 externally to the second, so as to appear in the same transverse sections. Rose (1898) 

 has fio-ured the same condition in a model of the tooth-germs of JDldelpliys aureus, 

 although he has mistaken the homologies in designating the external germ as second 

 instead of the third, and vice versa. Exactly the same conditions are seen in the adults 

 of all three families, Dasyuridse, Didelphyidse, and Peramelidse, where the second tooth 

 is inserted behind the first and the third tooth externally to the second, the tips at the 

 same time forming a uniform row. According to Dependorf's idea, the lower teeth of 



* As determined from their description of the tooth-development of Perameles (1897). 



