PURBECK FORMATIONS. 113 



of MyrmecoUm ; Siylodon has its analogue in Chrysodoris ; Peralestes has culminated in 

 Sarcophihs ; Triconodon in T/iylacinus ; Playiaulax is to Thylacoleo what the Weasel is to 

 the Lion. But derivative change has not advanced to the long-limbed saltatory herbivorous 

 type of Marsupial; nor has any evidence yet been had of a Mesozoic predecessor of the 

 climbing Koala, the volant Petaurist, or the burrowing Wombat. 



The American Mesozoic Marsupial (fig. 3, p. 20) exemplified the vegetative repetition 

 of many molars, significant of the same low comparatively unspecialised grade of Mammal, 

 evidence of which has been less sparingly, not to say more abundantly, got from European 

 deposits of corresponding geological age. 



The Marsupial type, through the operation, as I still believe, of some foreordained 

 natural law or secondary cause, has, in America, " progressed to, and been succeeded 

 by " ' the more specialised form of Didelphys ; in which, with reduction of molars to the 

 present type number, there has been a specialisation of some as " premolars,'' of others as 

 " true molars," strongly marked and carried to a degree far beyond anything which either 

 figures or descriptions allow us to infer in the diminutive many-molared Dromatherium 

 sylvestre. 



The derivative descendants of the ancient American primeval forms of Marsupialia 

 are reduced to that one genus or family called properly " Opossums " [Didelphys or 

 DidelpJiida) , now split up into such insignificant groups as PJiilaiider, Hemiurus, 

 MicrodelpJiys, Thylamys, Ckironectes, &c., all having essentially the same modification 

 and specialisation of Marsupial dentition as in Didelphys proper. 



It is remarkable and perplexing in the endeavour to conjecture out the operation of 

 the derivative law, to note, that the Mesozoic Marsupials of Europe, whose low, 

 comparatively non-diflferentiated condition has been sufficiently exemplified and dwelt 

 upon, have been also succeeded and finally represented in Europe by differentiated 

 carnivorous Climbers [Didelphidce) ; but on this continent they did not survive to enter 

 the latter half of the Neozoic period. Our latest European fossil Marsupial is a Miocene 

 Opossum. 



Does the present rich and varied condition of Marsupial life in Australasia exemplify' 

 the changes which lyencephalous mammalian organization in Mesozoic America and 

 Europe may have undergone or ramified into during the vast periods of time and changes 

 of terrestrial surface which have intervened between the formation of the coal-fields of 

 Virginia and its now growing gum-trees ; or between the Rhaetic breccias of Wirtemberg 

 and the lacustrine marls of Auvergne ? 



If Australia possessed Marsupials as far back in time as did America and Europe, 

 analogy would lead us to suppose that the primitive diminutive multimolar insectivorous 

 type prevailed. It has not there yet become extinct; but it seems to have been reduced 

 to the solitary exceptional form of the Myrmecohims. 



Let us suppose all the now existing Marsupials of Australia to have lived there, 



' ' Nature of Limbs,' p. 86. 



15 



