134 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



diprotodonty has been passed. Embryological evidence so far adduced 

 points to the conclusion that this is the case. Woodward's (1893) studies 

 of developing teeth in the Macropodidae led him to believe that the 

 large functional lower incisor is a persistent member of a series in which 

 it formerly stood in second place. Wilson and Hill (i 897) , from extensive 

 work on the development of Perameles, find that the first lower incisor 

 has been lost and that the one now found in terminal position is homo- 

 logous with the large lower incisor of the macropods. These authors say 

 (p. 509) : "It is interesting to compare the condition in Perameles with 

 that described by Mr. Woodward for Petrogale. In that form he found 

 two small calcified vestigial incisors. The anterior of these was entirely 

 in front of the large permanent lower incisor, which latter he therefore 

 regarded as really * 2. And may it not be that Woodward's 

 second vestigial incisor (his * 3 ) to which he was unable to recognize any 

 ancestor, is really the homologue of our dii whose legitimate successor 

 is Woodward's iz, the almost certain homologue of our l i\ ' of the adult 

 Perameles?" 



The dasyurids have the same number of lower incisors as Perameles, 

 but whether they also have lost the first pair has not as yet been demon- 

 strated. Embryological evidence in their case seems lacking. It was 

 formerly supposed (Cf. Thomas, 1887) that the dasyures had lost the 

 fourth incisor and therefore that the maximum of three incisors in higher 

 mammals represented the first, second, and third of the primitive series. 

 The evidence for this supposition adduced by Thomas now seems 

 inadequate, but in connection with theories of the origin of diprotodonty 

 it would be important to know whether the dasyurids have lost the same 

 incisor as the peramelids. 



The canines and premolars, which Bensley interprets as retrogressive, 

 constitute a similarity between Perameles and Ccznolestes and would seem 

 to favor a common ancestry rather than otherwise. Whether they are in 

 reality retrogressive is another question. Retrogression in this sense is 

 a sort of reversed homoplasy which can be recognized only if the imme- 

 diate ancestry of the form in question is known. That is, in the case of 

 Perameles, a didelphid ancestry is essential to a belief in the retrogressive 

 nature of its canines. On the other hand, if Perameles had no distinct 

 didelphid ancestry but descended from a more primitive form also 

 ancestral to the didelphids, it may have retained a primitive premolari- 

 form canine. Highly specialized canines appeared very early, in fact 

 they were well differentiated among some early reptiles, and various 

 Mesozoic mammals had simple single-rooted canines, but others, like 

 the Jurassic Triconodon, possessed double-rooted or grooved canines 

 similar to those now found in Perameles, Ccenolestes, and Myrmecobius 



