176 



herbivorous, possessed, in most cases, the full complement of teeth ; while forms cha- 

 racteristic of later times, such as the Felidse and Ruminantia, are remarkable for special 

 suppression of these organs. If the generalization were really of as wide an application 

 as has been claimed for it, we ought to find evidence of closer adherence to the general 

 archetypic model the further back we recede in time. But so far is Plagiaulax, at 

 present the oldest well-ascertained herbivorous mammal yet discovered, from giving 

 any countenance to the doctrine, that it actually presents the most specialized excep- 

 tion, so to speak, from the rule to be met with in the whole range of the Marsupialia, 

 fossil or recent. It had the smallest number of true molars of any known genus in 

 that subclass, six at least of the normal number of incisors being also suppressed"*. 



But Plagiaulax, viewed as a member of the same predaceous group of Marsupialia as 

 Thylacoleo, affords an interesting instance of adherence to the law above disputed. The 

 extinct pouched carnivore of the tertiary period shows a single carnassial tooth on each 

 side of the lower jaw ; the extinct pouched carnivore of the oolitic period retained in 

 one species three premolars of the carnassial type, in another species four (the normal 

 or type number) on each side of the lower jaw. The parallel runs very close with that 

 which the placental Carnivora show within the limits of tertiary time ; as when we com- 

 pare the miocene Hycenodon and its three lower carnassials with the modern Hyaena, 

 where they are reduced to one, or when we compare the miocene Amphycyon with its 

 three upper true tubercular molars with the modern Ursus, where they are reduced to 

 two, or the modern Felis, where they are reduced to one. If, also, the oolitic Phasco- 

 lothere, although it is known (to me) only by half its lower jaw and the teeth of that 

 moiety or " ramus," be compared with the modern Opossum, represented by the same 

 part, the more generalized type is conspicuous in the absence of the degree of differen- 

 tiation of the individual teeth in the oolitic fossil jaw which characterizes the homolo- 

 gous teeth in Didelphys. The canine is marked by only a slight superiority of size from 

 the antecedent teeth, which are of similar shape, and divided from each other by similar 

 intervals, in Phascolotherium. In Didelphys the canine is marked by greater relative 

 size and difference of shape from the close-set group of small incisors commonly anterior 

 to it. The seven molars in Phascolotherium show gradational differences of size, but 

 none of shape ; save some simplification of the two smallest, which are the first and the 

 last of the series of seven teeth. In Didelphys the last four molars are abruptly and 

 markedly differentiated from the three preceding ones, so that zoologists distinguish the 

 four as " true molars" from the three which are their " false molars." Phascolotherium 

 does nflt lend itself to this distinctionf . 



* Quart. Journ. Geol. Soc. vol. xiii. p. 276 ; XI. p. 427. 



f This well-known fact in comparative odontology is here repeated in reply to the question addressed by 

 Professor Hcxley, in his character of advocate of ' Uniformitarianism,' to the London Geological Society, 

 speaking for the then President, Mr. Leonard Horner: "in what circumstance is the Phascolotherium more 

 embryonic, or of a more generalized type, than the modern Opossum?" — Quarterly Journal of the Society, vol. 

 xviii. (1862) p. li. This is a mode of fence concealing individual knowledge and assuming general ignorance 

 of facts running counter to a view advocated by the rhetorician. 



