P. BECKLESII AND P. MINOR. 



427 



the ' Lethsea Geognostica,' as being probably a predaceons 

 marsupial (3rd edit. vol. ii. p. 122). 



The next point to which I would solicit attention is, that 

 Plagiaulax would seem in some respects to furnish a crucial 

 test of the soundness of certain generalizations which have 

 been put forward regarding the order of successive appear- 

 ance of mammalia upon the surface of the earth. It has 

 been maintained by British palaeontologists and comparative 

 physiologists l of the highest authority, that, while there is 

 no good proof of a serial progressive development from the 

 lower to the higher forms, there is evidence of another order 

 of development or successive passage, namely, from the general 

 to the special, as we descend from the oldest to the modern 

 period. It is urged by the advocates of this doctrine, that 

 the Mammalia of the Eocene period assimilated more to the 

 general plan of the archetype and to the embryonic con- 

 dition of the vertebrate organization ; while the Mammalia 

 of modern times successively furnish more and more numer- 

 ous examples of deviation from the archetype, all tending 

 towards special adaptation. Among other arguments, they 

 insist that the earliest Eocene Mammalia, both carnivorous 

 and herbivorous, possessed, in most cases, the full comple- 

 ment of teeth ; while forms characteristic of later times, such 

 as the Feliclw 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 coun- 

 tenance to the doctrine, that it actually presents the most 

 specialized exception, so to speak, from the rule, to be met 

 with in the whole range of the Marsupialia, 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 ; thus exhibiting, at the most 

 remote end of the chain, the very characters which, under 



1 Carpenter, Principles of Compar. 

 Physiology, 4th edit. 1854, pp. 107-111. 

 The doctrine here referred to is deve- 

 loped in considerable detail by Dr. Car- 

 penter in the passage above indicated. 

 In a note (loc. cit. p. 1 1 1 ) he disclaims 

 it as having originated with himself: 

 ' The principle expounded in this para- 

 graph has been prominently enunciated 

 and illustrated by Professor Owen in 

 various parts of his writings. The 

 remarkable facts here stated with re- 



spect to the dentition of mammalia are 

 contained in his article " Teeth," in the 

 Cyclopaedia of Anatomy and Physiology, 

 vol. iv.' Some interesting illustrations 

 bearing upon the retention of the typical 

 formula of dentition in the placental 

 mammals of the Eocene and Mioceno 

 periods, and upon the departure from it in 

 modern mammals, are adduced by Pro- 

 fessor Owen in his memoir ' On the Den- 

 tition of Phaeochcerus,' Phil. Trans, for 

 1850, p. 495.— H. F., June 20, 1857. 



