ITS DISPUTED AFFINITY. 439 



face, 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 channelled 

 sides meet in a finely serrated edge. Not the least remark- 

 able point in this striking list of agreements is the curious 

 numerical coincidence — these grooves being developed seven 

 in number, alike in the homologous premolars of PI. Bechlesii 

 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 minimum; in the 

 former the grooves are diagonal, in the latter vertical. With 

 this exception, and with some trivial details 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 Plagiaulax. The two convey to my mind the impres- 

 sion 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 Thylacoleo, a much larger predaceous marsu- 

 pial, 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. Now, 

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

 which can be settled by an appeal to the original speci- 

 mens. I have described the large premolar as essentially 

 alike in form, in the Kangaroo Eat and in Plagiaulax. Pro- 

 fessor Owen states that it is differently 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, 

 such as a modification in the outline of the same organ in 

 two nearly aUied forms, but a difference of type — or of ordinal 

 importance. 



Next as regards the assertion that in the general shape 

 the large premolar of Plagiaulax most resembles Thylacoleo. 

 For convenience, I separate the two terms of the comparison 

 in the sentence. Professor Owen has figured and described 

 the sectorial teeth of this large Marsupial, in his late memoir 

 on the 'Fossil Mammalia of Australia.' 1 In Thylacoleo the 



1 Phil. Trans., vol. cxlix. p. 318, Pis. xi. and xiii. 



