124 DR. B. A. BENSLEY ON THE EVOLUTION 



functional incisor formula to 1, the median teeth becoming greatly enlarged, while 

 the lateral ones, together -with the canines and anteriorly placed premolars, become 

 reduced or disappear. 



The fact has already been noticed that the insectivorous members of the Dasyurinse, 

 like the Didelpliyidte and many of the placental Insectivora, show a procumbent 

 development of the median npper incisors, which adds considerably to their functional 

 value as grasping and piercing organs in the capture of insect prey, but that, with the 

 partial exception of certain forms such as Thascogale penicillata and its prototype 

 _P. calura, they shoAV no corresponding differentiation of the median lower incisors; 

 further that in the placental SoricidtB the latter teeth are enlarged and elongated in 

 such a way as to act in concert with the median ujiper ones alter the manner of the tips 

 of a pair of forceps. The primitive members of the Phalangeridee {Acrobates, Distoe- 

 churus, Dromicia) show exactly the same differentiation of the median upper teeth as is 

 seen in tlie Dasyurin;e, combined with the same differentiation of the median lower ones 

 as is found in the Soricidae. This circumstance furnishes a complete clue to the origin 

 of the diprotodont modification, since it can be shown that all of the changes which take 

 ])l;ice in its formation are directly dependent on the differentiation of the median lower 

 incisors as grasping and piercing organs. 



It may be observed that none of the actually constructive stages in the development 

 of the diprotodont modiiication are illustrated in the existing Phalangeridie, the most 

 primitive condition found in that family being of the kind represented in PL 5. fig. 39, 

 of the dentition of Distoecliurits penual^is. Outside of the Australian group, however, 

 we find in the Epanorthidse and their allies much more primitive relations, and a com- 

 parison and figure of the dentition of Ccenolestes obscurus, the existing representative of 

 that family, has accordingly been introduced. 



If we suppose such a polyprotodont animal as Fhascogale penicillata to undergo a 

 further enlargement of the median lower incisors, the result would be twofold. In the 

 lirst place, the elongation of these teeth Avould be accommodated by a shortening of the 

 anterior portion of the lower jaw, and this Avould disturb the relations of all of the more 

 anteriorly placed lower teeth with the corresponding upper teeth. On comparison of 

 the figure of Coenolestes (PI. 5. fig. 38) *, it will be seen that, while the lower molars, as 

 well as the median and posterior premolars, occupy their normal positions with reference 

 to the corresponding upper teeth, all of the remaining lower teeth situate behind the 

 enlarged median incisors have been displaced backwards, and are now in a vestigial 

 condition. A continuation of the same development will be seen in Distvechurus 

 (fio-. 39), where, with a further shortening, at lea.st two teeth, probably representing the 

 canine and anterior premolar, have whoUy disappeared. In the second place, there 

 would be a reduction of the posterior ujjper incisors, since only those teeth with which 

 the tips of the median lower incisors come into contact could persist. Thus in 



* This specimen of Ccenolestes, kiudly lent to me hy Mr. Thomas, differs from the type specimen described by 

 him (189s h) in possessing an additional " intermediate" tooth on each side of the lower jaw. The full untemolar 

 lormula, which is thus 8 in this specimen, is greater than in any of the existiug Marsupials excepting the 

 Didelphyida;. 



