9G DE. B. A. BENSLET ON THE EVOLUTION 



tiicir tips are slightly pointed. In the species of Sminthopsis there is usually a more 

 or less marked tendency for the median upper teeth to be separated at their tips as 

 ■well as at their bases, and to project downwards rather than forwards. This 

 condition is to be regarded as special rather tlian as primitive. Dasjjnrns hallncatua 

 shows much the same condition as Phascogale, but following the remaining species of 

 Uasyurm we find the median upper incisors undergoing a gradual reduction by 

 Avliich tliev come to resemble the lateral teeth. In Sm^copldlus they are indistin- 

 guishable from the latter, except for their slightly more rounded section and their basal 

 separation. 



The differentiation of the median upper incisors in the smaller Dasyurinte represents 

 an insectivorous adaptation developed for the purpose of making the terminal teeth 

 more serviceable in the capture of insect-prey, and their subsequent retuin to a normal 

 spatulate condition, in the course of the carnivorous evolution, is due to the lapse of 

 tlieir original function. The reduction of these teeth is closely associated with a change 

 in the relative length of the incisor rows. In the smaller Dasyurin^e the latter are 

 relatively Ion"-, and meet in the middle line at an acute angle, so that the modified 

 median teeth are placed in the most advantageous position. Passing through the 

 species of Basyurus in the order above mentioned we find a successive shortening of the 

 incisor rows, until in SarcopJdlus they form an almost straiglit line across the front of 

 the muzzle. As mentioned below, there is also to be seen, in proceeding through the 

 same series, an increase in the functional importance of tlie canines, indicating a 

 gradual transference of the piercing and grasping functions from the median incisors 

 to the latter teeth. The shortening of the incisor rows is in part. connected with 

 the thickening of the muzzle for the support of the ealarged canines, but is more 

 especially developed for the purpose of placing the incisors in a position where they 

 will least interfere with the action of the canines, namely as close as possible to the 

 canine roots. 



Many of the placental Insectivora present a close resemblance to the Dasyurinae in 

 respect to the difi'erentiation of the median upper incisors. For example, in the 

 Soricidse, and in Centetes and Chrysochloris, the median upper incisors are much like 

 those of PhascogaJe, and the more specialized modifications of Sminthopsis are repeated 

 in the genera Echinops, Erimilns, and Limnogale (Centetidse), and also in Ilacroscelides 

 (Macroscelididte). It is interesting to note that, with the partial exception of such 

 forms as Fhascogale penicillata and its prototypal relative P. calura, the insectivorous 

 differentiation of the incisors in the Dasyurinse is confined to the median upper teeth, 

 but that in many of the Insectivora it extends to other of the incisors as well. More 

 especially is this noticeable in the Soricidse, where the median lower teeth are elongated 

 in such a way that they work against the median upper teeth after the manner of the 

 tips of a pair of forceps. This modification, while it is not indicated iu the Dasyurina?, 

 is highly characteristic of the Phalangeridse, and, as pointed out below, represents the 

 starting-point for the whole diprotodont modification characteristic of the last-named 

 family and its herbivorous derivatives. These facts taken together indicate that the 

 differentiated condition of the median upper incisors represents the first stage in the 

 insectivorous specialization of the incisors, and also that it represents a prototypal and 



