190 DR. B. A. BENSLET ON THE EVOLUTION 



Deciduons Premolars. — Except in the case of Myrmecohius, these teeth have not been 

 considered in dealing with the dentition of the Australian forms, the reason being that 

 their characters do not relate to the secondary evolution of the grou]^ *, but with the 

 primary differentiation of tlie Marsupials as a whole. They furnish such conspicuous 

 proof of the prototypal position of the Didelphyidse, however, that their characters may 

 well be used to supplement the evidence already given. 



It is a well-known fact that in the differentiation of the Marsupials one of the 

 dentitions has been obliterated, although as to which dentition is indicated is still a 

 matter of controversy. Wilson and Hill (1897), in expressing the opinion that it is the 

 milk-dentition \\hich has been reduced, have pointed out the fact that in the Didel- 

 phyidse the deciduous premolars are well developed, while in the Dasyuridae and 

 Peramelidaj they are reduced to vestiges. This they regard as indicating a final stage 

 in the obliteration of the milk-dentition, the posterior deciduous premolar being at the 

 present time in process of reduction. The following remarks, although specially 

 designed to show the prototypal position of the Didelphyidse, may be taken as con- 

 firmatory of that view. 



The deciduous premolars are ■well represented in the British Museum specimens of 

 the Didelphyidre, partly on accoimt of the completeness of the collection, and partly 

 from the fact that the teeth are not replaced until well on in life, and thus appear in all 

 of the younger specimens. They show very little variation throughout the family. The 

 lower teeth resemble almost exactly the true molars, and the same remark applies to 

 the upper teeth, except that here there is a fusion of the paracone with style b. The most 

 important feature of the teeth is, however, their large size and obvious functional 

 condition. They are at most only slightly smaller than the first true molars. 



The deciduous premolars are to be seen in only a few of the British Museum specimens 

 of Dasyuridae, as ISminthopsismurina, JPhascogale flavijjes, IP. minima, F. 2^fnicillata, and 

 Thylacinus cynocephalus. In all of these the deciduous teeth are found in an advanced 

 state of reduction. They are, in fact, quite vestigial. In all except Thylacinus they tend 

 to retain a molariform shape. There is an internal cusp, a large central cusp, a metacone, 

 and a third anterior smaller cusp, respectively representing protocone, metacone, and 

 piracone. The teeth are relatively more reduced in P. penicillata than in the smaller 

 forms. In Thylacinus we find an extreme stage of reduction, the teeth being represented 

 by minute triangular plates which are displaced before birth (c/. Plower, 1869). In 

 J)a.syurus, where the posterior premolars are absent, the deciduous premolars only occur 

 as calcified vestiges in the embryo (Woodward, 1895 ; Dependorf, 1898). 



Thomas (1887) has expressed the opinion that the reduction of the deciduous premolars 

 in the Dasyuridae is associated with the reduction of their successors, the posterior 

 premolars. While this is in a certain sense true, it must be remembered that the 

 association is only incidental. In Thylacinus, where the deciduous teeth are greatly 

 reduced, the permanent teeth are well developed and predominant. The same condition 

 is seen in the Peramelidae and in Phascolarctus. 



* A partial exception is afforded by the Macropodidae and advanced Phalangerinsp {v'lde infra). 



