MARSUPIALS 191 



gists would hesitate to refer this skull to the Diprotodontia ; but, remem- 

 bering the great geological age of the Plagiaulacidse and the probably 

 late origin of the true Diprotodontia, recognizing the marked differences 

 in the dental fonnula and form of the clieek teeth, and mindful of the 

 pitfalls of convergence and parallelism, especially among related groups, 

 we can only say that in Ptilodus the broad palate, with widely open 

 vacuities, the stout jugal reaching back to the glenoid, the inflexion of 

 the jaw — these are characters which undoubtedly betoken some degree of 

 kinship with the Marsupialia, but hardly prove that the Plagiaulacidae 

 are Diprotodontia in the strict sense. Indeed, Broom^^ has argued that 

 the Plagiaulacidae may prove to be an offshoot from the early Proto- 

 theria — that is, from the primitive group with Monotreme-like coracoids 

 and oviparous habits, which there is reason to believe^* was ancestral 

 both to the modem Monotremes and to the Marsupials. 



The extinct Patagonia n genus Polydolops and its allies, which were de- 

 scribed by Ameghino as Multituberculates, appear to differ from the true 

 Multituberculates in some important particulars, and the writer^^ has 

 adduced evidence tending to show that they are ^^pseudo-Multitubercu- 

 iates," derived perhaps from some early Epanorthids resembling Ah- 

 derites. 



Pantotheria 



With regard to the Mesozoic mammals of the order Pantotheria or 

 Trituberculata, which are often referred to the Marsupialia in the broad 

 sense, very little new material has come to light during the past ten 

 years; but these groups have been more or less reconsidered in their 

 bearing on the theory of trituberculy by Osborn^^ (1907), Gidley^^ 

 (1909), and the present writer^^ (1910). 



The origin of the so-called tritubercular molar has continued to chal- 

 lenge the ingenuity of authors. Gidle}^, developing Wortman's^® "pre- 

 molar analogy" theory, holds that the so-called protocone of the upper 

 molar has resulted froni the upgrowth of a basal cingulum, like the hypo- 

 cene, and that it has grown up pan passu with the talonid of the lower 

 molars, while the present writer^® thinks it more probable that in the 



" On Tritylodon. and on the Relationships of the Multituberculata. I'roc. Zool. Soc, 

 1910. pp. 76.3-768. 



1* Gregory, op. cit., pp. 157, 160, 148. 



« Ibid., pp. 211-214. 



" Evolution of Mammalian Molar Tooth, 8vo, New York, 1907. 



I'Proc. Washington Acad. Sci., vol. viii, 1906, pp. 91-110. 



18 Gregory, op. cit.. 1910, pp. 181-194. 



" Studies of Eocene Mammalia, etc. Amer. .Tour, Sci., vol. xiv, p. 94 ; ibid., vol. xvl, 

 pp. 215-218. 



2° Op. cit, pp. 184-189. 



