OF THE AUSTEALIAN MAESUPIALIA. 113 



cusps. The appearance presented in an extreme stage is well represented in PL 6. 

 fig. 15 of tlie lo\ver molar of Chaivopus castanotis. The same modification of otherwise 

 comparatively simple teeth is found in the Notoryctidse (PL 6. fig. 17 b) and in many 

 of the placental Insectivora. 



Incisors. — The most important featui'e of these teeth is the retention in all forms, 

 except P. Doreyanct and F. Cockerelli, of a fifth upper pair. In this feature, which is 

 also presented by Thijlacomys, the members of the family are more primitive than any 

 of the remaining Marsupials of Australia, but exactly resemble the Dideljihyidte. 



With the partial exception of the fifth upper pair, the incisors present a very 

 characteristic appearance on account of the great broadening of their tips. The median 

 upper teeth are not differentiated from the lateral ones as they are in the Dasyurinae, 

 the Didelphyidse, and the primitive Phalangeridae. This condition is apparently 

 secondary, as in Ilyrmecohliis and Notorijctes. The anterior four pairs of upper incisors 

 are usually seen to have their cutting-edges squarely truncated, but this condition is 

 probably the result of wear. In young specimens of Chceropus castanotis and JPerainelfx 

 nasnta the upper incisors show the same curious triangular shape as found in the 

 Didelphyidse and Dasyuridae {cf. text-fig. 2, f & g, p. 105). As already noted above, the 

 unworn teeth of P. obesula show a backward prolongation of the tips somewhat like 

 that found in My rmecobius. Tiie fifth upper incisors tend to be more or less caniniform, 

 like all of the ui)per lateral teeth of the latter form. 



The lower incisors are fairly constant in their characters. Their cutting-edges are 

 rounded and point to the type found in the Dasyuridae and Didelphyidje. The third 

 tooth shows a prominent posterior accessory cusp. This element probably represents a 

 secondary development. It is not present in the Didelpb.yida;, which have four lower 

 incisors, but is frequently present in the Dasyuridae, where, as in the Peramelidae, the 

 incisors have been reduced to three. 



The relative positions of the fifth ui:)per incisors and their occasional absence furnish 

 the first instance of the variability of the antemolar teeth already referred to. In the 

 otherwise primitive form P. BaJJrayaiia the muzzle is moderately elongated and the 

 fifth incisors are separated from the fourth by short diastemata ; in the intermediate 

 forms P. nasuta and P. Gumii the muzzle is excessively elongated and the diastemata 

 are correspondingly increased ; while in the final forms P. obesula and P. mucrura, as 

 also in Chavopus castanotis, the muzzle is again shorter and the diastemata are scarcely 

 in evidence. Again, of tlie two [)rimitive forms P. Doreyana and P. llaffrayana, the 

 fifth upper incisors are present in one and absent in the other, while they are present iu 

 all of the specialized forms with the exception of P. Cockerelli. These characters may be 

 taken, in connection with certain others of the canines and premolars, as showing that 

 the sequence indicated by tlie modifications of the molars cannot be relied ujjon as 

 indicating the true relationships of the various species. 



Canines. — These teeth are even more variable in their characters than are the fifth 

 vipjjcr incisors in their position. In P. liaffrayanu tlie u])per tooth is short, moderatelv 



