PURBECK FORMATIONS. 95 



species has left me under the conviction that what has been above submitted to 

 Palseontologists in the attempt to determine the affinities of Plagiaulax is near the truth : 

 and that having the lower jaw and its entire dentition to work from, the Palaeontologist 

 ought to be able to indicate, approximatively, its place in the Natural System. 



I should not have presumed so far if my material had been a solitary premolar ; 

 supposing even that the last and largest of the series had been the sole indication of 

 what we now know as VlagiauJax. 



If it had resembled a premolar of a Rat-kangaroo as much as it diflPers from one ; if its 

 crushing edge had been straight and vertically notched, instead of being curved and 

 obliquely notched;, or ' serrate ;' if its ridges had been vertical instead of oblique ; no infer- 

 ence as to the number and kind of teeth with which such premolar had been associated, in 

 the otherwise unknown Oolitic beast could be safely or scientifically drawn ; still less could 

 the Palaeontologist be justified in jumping at the conclusion that the old user of this solitary 

 evidence of its dental tools had been a saltatory herbivore ! If one desired to have it 

 believed that a Macropodal or Poephagous Marsupial had existed in Triassic or Oolitic 

 antiquity, he might indeed substitute for scientific reasoning confidence of assertion.^ 



With a fossil premolar as like that of Hypsi/prymnus as the last premolar of Playiaulax 

 is unlike, — I will not insult the common sense of Zootomists by citing the microlestian 

 denticle (PI. I, fig. 16) as a parallel case, — the competent Palaeontologist viewing such 

 premolar would call to mind instances where similar premolars are associated, in the 

 Mammalian class, with very diflierent molars, canines, and incisors. A premolar does a 

 part, but not the whole, nor commonly the main work, of the preparation of the food for 

 deglutition and digestion, &c. A premolar may show, as in Hypsiprynmus and Playiaulax, 

 an admirably fitting instrument for dividing by cutting or by sawing. But such a fossil 

 instrument cannot, by itself, teach the nature of the substances to the division of which it 

 was applied by the living animal ; still less can it justify a conclusion as to the kind of 

 locomotion with which the beast carried itself to its food or prey. 



The great master and founder of Palaeontology has been held by some to have 

 hazarded over much in estimating the amount of inference that could be drawn from a 

 solitary fossil tooth. But this at least all subsequent experience has confirmed, that he 

 selected the class of teeth which best justifies his axiom. I proceed, next, to consider the 

 physiological deductions which may be drawn from our knowledge of these light-giving 

 elements in the dentition of Playiaulax. 



1 Thus, Mr. Boyd Dawkins affirms : — " The presence of the Macropoda (Van der H. ^ Poephaga, Owen) 

 is proved by the discovery of the Kangaroo-rat allies : namely, in the Purbeck beds, of the Plagiaulax, the 

 true affinities of which have been so amply demonstrated by Dr. Falconer ('Quart. Journ. Geol. Soc.,' 

 vol. xiii, p. 261 ; vol. xviii, p. 348) ; in the Rhsetic bone-bed, of the Microlestes of Frome and Diegerloch, 

 closely allied, according to Professor Owen, to Plagiaulax ('Palseont.,' p. 303) ; and, lastly, in the strata 

 below the bone-bed, by the discovery of the Hj/jjsiprymnopsis Rhceticus of the Watehet shore." 'Quarterly 

 Journal of the Geological Society of London,' vol. xx (1864), p. 412. 



