276 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Mar. 11, 



anything fossil or recent known to him*. Pictet, in his * Paleonto- 

 logie,' doubtingly includes Microlestes among marsupials, for reasons 

 which he does not state, upon the authority of some of the German 

 describers, whose memoirs I have not been able to consult. Bronn 

 notices Microlestes, in the ' Lethsea Geognostica,' as being probably 

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



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

 giaulax 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 appearance of mammalia upon the 

 surface of the earth. It has been maintained by British palaeontologists 

 and comparative physiologists f 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 condition of the vertebrate organization ; while 

 the Mammalia of modern times successively furnish more and more 

 numerous examples of deviation from the archetype, all tending to- 

 wards special adaptation. 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 

 characteristic of later times, such as the FelidcB and Ruminantia, are 

 remarkable for special suppression of these organs. If the generaliza- 

 tion were really of as wide an application as has been claimed for it, 

 we ought to find evidence of closer adherence to the general arche- 

 typic model the further back we recede in time. But so far is Pla- 

 giaulax, at present the oldest well-ascertained herbivorous mammal 

 yet discovered, from giving any countenance 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 the generalization in 

 question, we might ii priori have expected to encounter at the near 

 end, among existing marsupials. 



* Cited in Lyell's Manual, 5tli edition, p. 343. 



t Carpenter, Principles of Corapar. Physiology, 4th edit. 1854, pp. 107-111. 

 The doctrine here referred to is developed in considerable detail by Dr. Carpenter 

 in the passage above indicated. In a note {loc. cit. p. Ill) he disclaims it as 

 having originated with himself: " The principle expounded in this paragraph has 

 been prominently enunciated and illustrated by Professor Owen in various parts 

 of his writings. The remarkable facts here stated with respect 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 reten- 

 tion of the typical formula of dentition in the placental mammals of the Eocene 

 and Miocene periods, and upon the departure from it in modern mammals, are 

 adduced by Professor Owen in his memoir ' On the Dentition of Phacochoerus,' 

 Phil. Trans, for 1850, p. 495.— H. F., June 20th. 



