116 



DE. B. A. BEXSLEY ON THE EVOLUTION 



It will be seen that both in Terameles obesula and in Thylacomys leucura the upper 

 molars are functionally quadrituberculate, tlie main cusps being in the former the 

 protocone, liypocone, paracone, and metacone, but in the latter the protocone, metacone, 

 and styles h and c. The upper molars of the Phalangeridse present in most cases an 

 actually quadrituberculate pattern, which is ancestral to the advanced patterns of the 

 ]\[acropodida3, Phascolomyidaj, and Diprotodontida3. The question therefore arises as 

 to whether the four cusps of the Phalangeridse are homologous witli those of Perameles 

 or those of Thylacomys. 



In the more primitive forms of the Phalangeridse [cf. PI. 5. figs. 14 & 15, Dromicia 

 concimia, Petaurus sclureus) the upper molars show a vestigial external cingulum, whicli 

 appears to be equivalent to that bearing the external styles in polyprotodont forms, and 

 in one section of the family (Phascolarctinas) vestigial styles are actually present. This 

 indicates that, although resembling them in many respects, the two outermost of the 

 main cusps in the Phalangerida? are not equivalent to the modified external styles in 



Is'ormal and aljerrnnt production of quadrate upper molars in Marsupials and Placentals. 



riacentals : — A. Proiogonodon ; B. Euprutor/oiiia ; C. I'heiiacodus (Condylarthra) ; D. Coryplwdon (Amblypoda). 

 Marsupials : — E. Perameles Doreyana ; F. P. BougainviUn ; G. P. ohesida ; H. Thi/lacomi/s. Abbreviations : 

 pr., protocone; pa., paracone ; me., metacone; hy., hypooone ; ps., parastyle ; st.b., style b ; st.c, style c. 



Thylacomys. Purthermore, in the primitive Phalangeridne (cf. PI. 5. fig. 13, Dlsta'- 

 churns pennatvs) the postero-internal cusp is relatively small, but increases in importance 

 in the more advanced stages (fig. 16, Trichosurus vulpecida) in precisely the same way 

 that the hypocone does in the PerameUnae, and is therefore more probably homologous 

 with that cusp than with the displaced metacone of Thylacomys. Finally, in the 

 Placentalia the formation of a quadrituberculate tooth by the addition of a hypocone 

 to an originally triangular crown is the normal procedure, since it is indicated in various 

 members of the Condylarthra, Primates, Inseotivora (Soricidse, Macros celidse, Erina- 

 ceidse). The formation of a quadrate tooth by displacement of the metacone is rare, 

 being only found, so far as the writer is aware, in the Amblypoda. The significance 

 of this will be more apparent from the accompanying diagrams (text-fig. -4), in which 



