chap, ix.] TEETH : PHALANGERIDJE. 249 



In these forms there is only one tooth having a milk -pre- 

 decessor, and in all the genera here referred to this is a distinct 

 and recognizable tooth, with a chisel-shaped crown. Following 

 Thomas' system I shall call this tooth p 4 throughout. This name 

 is used as being well understood and convenient, but without 

 any intention of subscribing to the principles of homology upon 

 which the system of nomenclature is based. 



In front of p 4 there is great diversity. 



In Thomas' paper 1 a careful and well-considered attempt was 

 made to bring these anterior teeth into a formal scheme of homo- 

 logies, and though the application of this method to the teeth 

 of the lower jaw was avowedly tentative, yet at first sight the 

 results in the case of the upper teeth were fairly satisfactory. 

 Nevertheless it appears to me that in view of the facts of Variation 

 about to be related, the system elaborated by Thomas breaks 

 down; not because there is any other system which can claim 

 to supersede it, but because the phenomena are not capable of 

 this kind of treatment. To anyone who will carefully study the 

 examples given in the following pages, especially those relating 

 to the genus Phalanger, it will, I think, become evident that it 

 is not possible to apply any scheme based on the conception that 

 each tooth has an individual Homology which is consistently 

 respected in Variation. 



The evidence concerns first the premolars of the upper jaw, and 

 secondly the lower " intermediate " teeth. Inasmuch as in several of 

 the cases there was Variation in both these groups of teeth, the evidence 

 relating to them cannot well be separated. As regards the upper 

 teeth, all the cases of importance occurred in Phalanger and Trichosurus, 

 and owing to the similarity between the dentitions of these two genera 

 it is not difficult to emj)loy terms which shall be distinctive, though 

 the question of the homologies of the teeth go unanswered. In all 

 the forms concerned there are three upper incisors, and the tooth 

 immediately succeeding them will be called the canine, though its 

 position and form diner greatly in the various genera ; for while in 

 Phalanger and Trichosurus it is a large caniniform tooth placed on the 

 suture between premaxilla and maxilla, in Pseudochirus, for instance, 

 it is proportionally smaller and stands in the maxilla at some distance 

 behind the suture. 



Upper jaw. As already stated, the large premolar having a milk- 

 predecessor will be called p*. 



In Trichosurus between the canine and p 4 there is usually one 

 large tooth, in shape and size much like the canine : this tooth will be 

 called F 1 as Thomas proposed. Though when present it is large, it is not 

 rarely absent altogether (v. infra). In Phalanger there is a similar^ 1 , 

 though of somewhat smaller size ; but besides X there is usually another 

 premolar, a small tooth, placed between l^_ and jp\ On Thomas' system 

 this is F J and for purposes of description the name will be used here. 

 In the left upper jaw of the skull shewn in Fig. 65 C, if, if and P^ are 



1 Phil. Tram., 1887, clxxviii. and Cat. Marmp. Brit, Mus. 



