lxXXviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



fromplagios, " oblique," and aidax, " groove," in reference to the dia- 

 gonal grooving of the premolars) has been established by Dr. Falconer; 

 and he has been already enabled to distinguish two species, one named 

 P. Becklesii in commemoration of the zealous discoverer of the speci- 

 mens upon which it is founded, and P. minor. Dr Falconer points 

 out that the coronoid process in Plagiaidax resembles more that of 

 the predaceous Marsupials, and especially of the Ursine Dasyurus, 

 than it does that of the herbivorous families, differing in a marked 

 degree from the elevated strap- shaped coronoid of Hypsiprymnus, 

 though at the same time being less elevated than in the preda- 

 ceous genera whether marsupial or placental; but, after a more 

 careful investigation of the question of affinity in every direction, he 

 concludes that Plagiaidax may be considered as a marsupial form of 

 Rodent, constituting a peculiar type of the family to which Hypsi- 

 prymnus belongs. The genus must have presented a form to which 

 there is nothing exactly similar in living marsupials. 



It may, he observes, for aught which can be asserted to the contrary, 

 have had the volant habits of the Flying Phalangers, and have flitted 

 from tree to tree among the Oolite forests by means of parachute-like 

 folds of the skin. The species were probably herbivorous or frugivo- 

 rous like the Kangaroo-rats ; but there is nothing in their teeth to 

 show that they were either insectivorous or carnivorous. The largest 

 species was about the size of a squirrel, the other much smaller. 

 Professor Owen has designated another form by the generic name of 

 Triconodon, so that at present there Are three described Mammalian 

 Purbeck genera — Spalacotherium, Triconodon, and Plagiaidax. Dr. 

 Falconer also points out the remarkable resemblance between the 

 molar teeth of Plagiaulax minor, and those of the Triassic genus 

 Microlestes antiquus of Plieninger ; and this resemblance may help to 

 settle the true character of the Microlestes, supposed by some to be 

 predaceous, by others to approximate to an omnivorous or omni- 

 voro-insectivorous type. 



Dr. Falconer further remarks in respect to the speculative views of 

 palaeontologists, — that, whilst they do not consider that there is any 

 satisfactory evidence of a progressive serial development from the 

 lower to the higher forms, there has been another form of serial pro- 

 gression, namely, from the general to the special, the animals of the 

 older period being more perfect in respect to an archetype, or, as it may 

 be called, a normal type, whilst by degrees there is a divergency from 

 this archetype, in order to assume a more special character, and to pro- 

 gress towards a special adaptation to new circumstances or conditions 

 of life. Now Plagiaulax is, as Dr. Falconer observes, the oldest her- 

 bivorous mammal yet discovered ; and yet, so far from adhering to the 

 general archetype, it is far more specialized than are any of the 

 Marsupials, whether fossil or recent, exhibiting characters, at the 

 earliest epoch, which ought rather to have been found in animals 

 of the existing epoch, — a fact, therefore, which is entirely at va- 

 riance, in his opinion, with the theoretical views to which I have 

 alluded in speaking of the paper of Professor Owen. The case is 

 one of difficulty ; and I will only repeat that it cannot be explained 



