112 GEOLOGICAL DISTRIBUTION 
distinguished by the convexity of the cutting edge and their oblique 
grooving. This remarkable and highly specialised type has been the 
occasion of one of the most interesting discussions on the inferences 
which may be drawn as to the affinities and habits of an otherwise 
unknown animal from the structure of a small portion of its organisa- 
tion which occurs in the annals of natural history—a discussion 
carried on with great ability, ingenuity, and wealth of illustration 
on both sides. Dr. Falconer maintained that it was more nearly 
allied to the Rat-Kangaroo (Potorous or Hypsiprymnus) than to any 
other existing form, and that, as it is known that these animals 
feed upon grass and roots, “it may be inferred of Plagiaulax that 
the species were herbivorous or frugivorous. I can see nothing in 
the character of their teeth,” he adds, “to indicate that they were 
either insectivorous or omnivorous.” Sir R. Owen, on the other 
hand, from the same materials came to the conclusion that ‘the 
physiological deductions from the above-described characteristics of 
the lower jaw and teeth of Plagiaulaz are that it was a carnivorous 
Marsupial. It probably found its prey in the contemporary small 
insectivorous mammals and Lizards, supposing no herbivorous form 
like Stereognathus to have co-existed during the Upper Oolitic 
period.” 
It is impossible here to give at any length the arguments by 
which these opposing views are respectively supported, but it may 
be indicated that the first-mentioned is strongly countenanced by 
the consideration of the following facts: (1) all existing Marsupials 
may be divided, so far as their dentition is concerned, into two 
groups—(a) those which have a pair of large more or less procumbent 
incisors close to the symphysis of the lower jaw, and rudimentary 
or no canines (diprotodont dentition), and (#) those which have 
numerous small incisors and large pointed canines (polyprotodont 
dentition) ; (2) the vast majority of the former group are purely 
vegetable feeders, and almost all of the latter are carnivorous or 
insectivorous ; and (3) Plagiaulaa, so far as its structure is known, 
shows an analogy with the former group; and, as we have no sure 
basis for inferences as to the habits of an unknown animal, but the 
knowledge of the habits of such as are known, we have no grounds 
for supposing that its habits differed from those forms having an 
analogous type of dental structure.1 
Allied types, such as Ctenacodon, are also met with in the Upper 
1 The whole discussion is contained in the following memoirs: (1) H. 
Falconer, ‘‘ Description of Two Species of the Fossil Mammalian genus 
Plagiaulax, from Purbeck,” Quart. Journ. Geol. Soc. vol. xiv. 1857 ; (2) B. Owen, 
art. ‘‘ Paleontology,” Encyclopedia Britannica, 8th ed., 1859; (3) H. Falconer, 
“On the Disputed affinity of the Mammalian genus Plagiaulax,” Quart. Journ. 
Geol. Soc. vol. xviii. 1862; (4) R. Owen, ‘‘ Monograph of the Fossil Mammalia 
of the Mesozoic Formation,” Paleontographical Society, 1871. 
