256 
PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
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 , w 7 here they are reduced to one. If, also, the oolitic Phasco- 
l other e, 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 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 Didelpliys 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 
not lend itself to this distinctionf. 
A still more generalized type of dentition is shown by the multiplication of slightly 
differentiated teeth in the genera Ampliitherium , Ampliilestes , Spalacotherium, Peralestes, 
Stylodon , &c., of the lower and upper oolites. One solitary form ( Myrmecobius ) alone 
remains at the antipodes with minute and slightly differentiated teeth, in number ex- 
ceeding the type one in most modern Mammals, and recalling that in lower and wider 
vertebrate groups. 
The two or three smaller but functional premolars in advance of the large lower car- 
nassial in the mesozoic Plagiaulax are reduced to two more minute functionless and 
speedily lost premolars in the neozoic Thylacoleo. 
* Quart. Journ. Geol. Soc. vol. xiii. p. 276; XI. p. 427. 
t This well-known fact in comparative odontology is here repeated in reply to the question addressed by 
Professor Huxley to the London Geological Society : “ 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. 
