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 (Potormis 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 Plagiaulax 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 (b) 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) Plagiaulax, 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) R. Owen, 

 art. " Palaeontology," Encyclopaedia 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," Palceontographical Society, 1871. 



