110 DR. B. A. BENSLET OX THE EVOLUTION" 



that, among tlie Placentals, the Condylarthra do with reference to the Creodonta and 

 the Ungulata respectively. It is also an interesting fact that among the existing 

 Insectivora, a grouj) showing a fairly wide range of dental differentiation, we find in the 

 Soricidae molar patterns of an incipient omnivorous type which parallel almost exactly 

 tlioseof the Peramelidaj, just as in the Talpidse and Clirysochloridse we find insectivorous 

 modifications paralleling those of the Dasyuridae and Notoryctida3 respectively. 



Unlike that of the Dasyurinai, tlie dental evolution of the jiresent family does not 

 represent a uniform progression ia one direction. In the members of the dominating 

 genus Perameles the molars will be seen to present a progressive omnivorous develop- 

 ment, associated Avith an increasing hypsodontism, which is quite as gradual as the 

 carnivorous development of the Dasyurinse; while the anteuiolar teeth, on the other 

 hand, will be seen to present a varied assortment of local and more or less retrogressive 

 characters. The cause of this condition may be found in the jieculiar manner in which 

 the family has been derived. The ancestors of the Peramelida? were arboreal animals, 

 and were probably either purely insectivorous or at most only slightly omnivorous. On 

 becoming terrestrial they have gradually become moi'e completely omnivorous, as shown 

 by the successive modifi(5ations of the molars. But they have also become fossorial to a 

 varying extent, and this development appears to represent the prime cause of the 

 antemolar variation. The hypsodontism of the molar crowns represents a compensatory 

 development, the object of which is to countei-act the effects of the mechanical wearing 

 doAvn of the teeth caused by the presence of particles of earth in the food. It owes its 

 uniformity to its association with the omnivorous development of the molars. The local 

 and retrogressive characters of the antemolar teeth may be due to a slight extent to the 

 reduction of their function in the omnivorous evolution, but are more closely connected 

 with the displacement of these teeth from their original relations by a varying elonga- 

 tion of the muzzle. The. latter is here, as in many other cases, the result of fossorial or 

 semifossorial habit. 



The family may be divided on a basis of molar characters as follows : — 



A. Upper molars normal ; triangular, becoming quadrate by the development of a 



hypocone. External styles not conspicuously enlarged Peramslinge. 



Geuera : Peranides, Chmropus. 



B. Upper molars aberrant ; quadrate through displacement inwards of tlie meta- 



coue ; external styles greatly enlarged and foi'ming with the protocone and 



metacone the main cusps of the crown Thylacomyinae. 



Genus : Thylacomys. 



Some doubt may be expressed as to the advisability of making a major division in the 

 Peramelid^ on account of the fact that the family is a small one and the three genera 

 closely related. The type of molar modification found in Thylacomys is, ho\\'ever, 

 unique in the Marsupials, and rare in the Mammals generally, so that this form seems 

 fairly entitled to distinction. 



