PEOFESSOE OWEN ON THE FOSSIL MAMMALS OF AUSTEALIA. 
255 
Taking the same range of the molary series for comparison as in Thylacoleo and ex- 
isting Diprotodonts, in reference to the character of size of the last trenchant premolar, 
the tooth equals in antero-posterior extent one-half of that series in Plctgiaulax. But 
in this more ancient Diprotodont the premolars anterior to the last large one have not 
undergone the extreme degradation which they show in the tertiary fossil ( Thylacoleo ) 
and in some existing Diprotodonts of Australia. They are modified, in Plagiaulax , for 
sectorial function, and are so combined with the last and largest sectorial as to work with 
it as one instrument, obliquely ridged and notched at the convex cutting-margin, like a 
section of a circular saw. I have elsewhere * pointed out the advantage of this modifi- 
cation of carnassial in dividing the integuments and other tissues, tougher and drier 
than those in Mammals, of the lacertian members of the cold-blooded class which so 
abounded with the small carnivorous Marsupials in the same Mesozoic period and place. 
If it be admitted that, so far as the lower jaw and its dentition show, Plagiaulax 
(figs. 10 & 15), with its two or three reduced anterior premolars, its suddenly enlarged 
hind premolar, its disproportionately small and few (two) tubercular molars, and its 
large laniarifonn upcurved incisor, comes nearest to Thylacoleo (figs. 8 & 14), it is plain, 
from the antecedent comparisons with existing Diprotodonts, that there are no grounds 
for inferring the Macropoda to have been derived from the Paucidentata , or these from 
Rat-kangaroos. 
What we do learn from consideration of the fossils in question is, the fact of an addi- 
tional and most interesting modification of the Diprotodont section of the Marsupial 
order or subclass, unknown before the discovery of these fossils^ We further learn that 
such modification, which, from the extreme reduction of the true molar series, I have 
been led to take as the character of a “ paucidentate” family of Marsupials, was 
already established at the Purbeck period ; yet with modifications interestingly exem- 
plifying the tendency to the “more generalized condition of structure” as compared 
with the newer tertiary extinct form. 
§ 17. Tendency from the general to the particular in the Dentition of the Paucidentate 
Marsupials. — But I am here met by another objection. Dr. Falconer, attacking the 
principle of the tendency to transition in organisms from generalized to specialized 
structures as they approach in geological position the present time, writes : “ Among 
other arguments, they insist that the earliest Eocene Mammalia, both carnivorous and 
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 , 
Owen, Monograph on Mesozoic Mammals, tom. cit. t. iv. figs. 9, 12. 
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