PURBECK FORMATIONS. 89 



No existing Diprotodont offers the mandibular formula of Plagiaulax. In the 

 Phalangers a subtrenchant tooth (fig. 9, jo 4), in contact with the true or tubercular 

 molars, and with a crown reaching to the same level, is 

 reckoned as the last or hindmost premolar, and as the ^^' 



homologue of the similarly developed and situated tooth 

 in Phascolardos and in most ' Poepliaga! ^ I view the 

 last and largest of the premolars of Plagiaulax as the 

 homologue of this tooth, and symbolise it as^ 4. 



In Phalangista Cookii (fig. 9) three small teeth 

 intervene between the last premolar {p 4) and the in- 

 cisor [i). Two of them are held to bejy 3 and jo 2— the *^'"'' pL^at^L^cSr''*'"' 

 homologues of those so symbolised in Plagiaulax 



Becklesii (fig. 8). With respect to the anterior small tooth, it may be questioned whether 

 it be the homologue of ^ i, in Plagiaulax minor, or the rudiment of a lower canine. At 

 any rate, as regards number of mandibular teeth between the incisor and the true molars, it 

 is only in the Carpophagous family of Diprotodonts that species are known corresponding 

 with the species of Plagiaulax. In Pefaurus {Belideus) jlaviventer four denticles intervene 

 between the functional premolar and the incisor.^ 



Some zoologists have founded subgeneric divisions, with names, on the difference in 

 number of the small premolars, and would, on like grounds, place in distinct genera 

 Plagiaulax BecHesii and Plagiaulax minor. A better ground for svich distinction is 

 afforded, among existing Carpophaga, by the small volant species, Petaurus {Acrobata) 

 pygmceus, in which the true or tuberculate molars are reduced to three on each side of 

 both jaws ; whilst between these and the incisor in the lower jaw are interposed four teeth ; 

 add thereto the shape of the last premolar, which has exchanged the trenchant for the 

 acuminate character. Mere form, however, of one or more premolars is not enough to 

 determine the reference to, or the removal from, such a group as Carpophaga of a species 

 proved by more important characters to belong to that group or to one of like value in 

 the diprotodont series. 



In some Poephaga, for example, the tooth answering io p 4 in Plagiaulax resembles 

 it in size and trenchant shape, being also grooved ; yet the Poephaga depart further than 

 do most Carpophaga, in having no tooth interposed between p 4 and the large procumbent 

 incisor of the lower jaw. 



The Hypjsiprymnidce or Rat-Kangaroos, with a ridged and trenchant p 4, have it fol- 

 lowed by four molars with massive triturating crowns. Of these the first three have " a 



' This term, applied in 1839 to the diprotodont family including Hypsiprymnus, Macropus, and their 

 subgenera, is preferable to the subsequently propounded one, Macropoda, of Van der Hoeven ; because the 

 latter is equally applicable in its descriptive sense to the long-legged saltatory Polyprotodonts. See Owen, 

 Classification of the Marsupialia, in 'Transactions of the Zoological Society of London,' 4to, vol. ii, p. 315. 



2 See Art. Marsupialia, ' Cyclopaedia of Anatomy,' vol. iii, 1841, p. 264, fig. 89. 



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