1862.] FALCONER PLAGIALTLAX. 355 



in the dentition of the fossil genus. In P. BecMesii there are three, 

 and in P. minor, four of these teeth, which diminish rapidly in size 

 from the last to the iu'st *. I here take the last as the most deter- 

 minate in form, and in its natui-e the most constant. I compared it 

 rigorously ^Tith the corresponding tooth of Hypsiprymnus Gaimardi, 

 and I affirm now, as I did in my original paper, that these homo- 

 logous teeth, in the two genera, are identical in every essential point 

 of form and construction. In proof, I refer to figures 5 and 6 of the 

 representations above cited, the former showing the last premolar of 

 Plagiaulax, the latter of Hypsiprymnus. The resemblance is so 

 manifest and direct, that I never contemplated that it could be called 

 in question ; but, as it has been questioned, it is necessary to descend 

 to particulars. In both, the crown viewed from the side is of a 

 quadi'ately oblong form, the length exceeding the height ; in both, it 

 is compressed and trenchant, the sides sloping uniformly from the 

 base to a thin edge like a wedge ; in both, the basal part of the tooth 

 presents a smooth surface, above which the crown is traversed by a 

 series of close- set, uniform, and exquisitely defined parallel grooves, 

 sharply angular, and bounded by linear ridges ; in both, these grooves 

 occupy both sides of the tooth ; and in both, the channeled sides 

 meet in a finely serrated edge. Not the least remarkable point in 

 this striking list of agreements is the curious numerical coincidence, 

 — these grooves being developed seven in number, alike in the homo- 

 logous premolars of PI. Beclclesii and of Hypsiprymnus Gaimardi. 



As to the points of difference : in Plagiaulax there are three or 

 four of these teeth, while in Hypsiprymnus there is but one ; in the 

 former, they are presented with the maximum of development, in the 

 latter with the ^ninimum ; in the former the grooves are diagonal, in 

 the latter vertical. With this exception, and with some trivial de- 

 tails of difference in the proportion of the length of crown to its 

 height, and in the amount of the basal surface free from grooving, 

 the last premolar in. Hypsiprymnus is identical in its characters with 

 that of Plagiaidax. The two convey to my mind the impression of 

 being typically alike. 



The objects strike Professor Owen in a very different light. His 

 statement is that, '^ in the general shape and proportions of the large 

 premolar and succeeding molars, Plagiaulax most resembles Thyla- 

 coleo, a much larger predaceous marsupial, from the tertiary beds in 

 Australia. But the sectorial teeth in Plagiaulax are more deeply 

 grooved ; whence its name. The single compressed premolar of the 

 Kangaroo-rat is also grooved ; but it is differently shaped," &c. 

 ISTow, apart from the inferences, here is a conflict of description, 

 which can be settled by an appeal to the original specimens. I have 

 described the large premolar as essentially alike in form, in the Kan- 

 garoo-rat and in Plagiaulax. Professor Owen states that it is dif- 

 ferently shaped in the two : if so, I invite him to show wherein the 

 difference consists (I have failed to detect, and he as yet to indicate 

 it), — bearing in mind that here it is not a question of slight difference, 



* See Quart. Journ. Geol. Soc. vol. xiii. pp. 278-281, figs. 1-15. 



VOL. XVIII. PART I, 2 B 



