356 PEOCEEDIN&S OF THE GEOLOGICAL SOCIETY. JuHC 4, 



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

 allied forms, but a difference of type — or of ordinal importance. 



Next as regards the assertion that in the general shape the large 

 premolar of Plagiaulaoc most resembles Thylacoleo. Por convenience, 

 I separate the two terms of the comparison in the sentence. Pro- 

 fessor Owen has figured and described the sectorial teeth of this 

 large Marsupial, in his late memoir on the " Possil Mammalia of 

 Austraha"*. In Thylacoleo the inferior premolars are reduced 

 to a single, but enormously large and massive, carnassial, with 

 two small tubercular teeth behind it. This carnassial (figs. 16-19) 

 consists of a long blade, high in front and lower behind, so that, if 

 notched in the middle, the divisions would in some degree resemble 

 the anterior and posterior lobes of the corresponding tooth in the 

 placental Carnivoraf; and the worn summit is distinctly concave 

 lengthwise : conversely, in both species of Plagiaulax the cor- 

 responding tooth is convex, and the outline of the whole series de- 

 scribes a convex curve, of which the last premolar forms the most 

 salient part. The base of the carnassial in Thylacoleo is " slightly 

 grooved vertically" on the inside (fig. 16). These indentations dis- 

 appear about half-way up towards the edge, where the surface be- 

 comes reticulately rugose, being precisely the reverse of what occurs 

 in the last premolar of Hypsiprymnus and Plagiaulax. Besides the 

 difference of their position upon the tooth, the grooves of the car- 

 nassial of Thylacoleo present the appearance of furrows, separating 

 superficial undulations of the enamel. A transverse section of the 

 basal part of the crown would yield a faintly crenated outline, wholly 

 different from the salient and reentering angles of the close-set 

 parallel grooves of Plagiaulax and Hypsiprymnus. These undulations 

 are exhibited chiefly, if not solely, on the inner side ; their presence 

 on the outer is not mentioned. Further, if the indentations on the 

 premolar of Thylacoleo are to count for anything as signifixcant of 

 affinity, it should be with Hypsiprymnus rather than with Plagi- 

 aulax, since the furrows are vertical in the two former. In fact, in 

 the outline and proportions of the vertical section, the premolar of 

 Thylacoleo differs less from Hypsiprymnus than it does from that of 

 Plagiaulax. I have failed to realize the asserted resemblance be- 

 tween Plagiaulax and Thylacoleo in the form of the last premolars ; 

 and in the details of outline, section, curvature of edge, crenulation, 

 surface-markings, &c., I am more impressed with the differences 

 than with any one point of agreement. 



Let us now consider the inference as to the function of these teeth. 

 It is expressed thus : — " The large front tooth is formed to pierce, 

 retain, and kill : the succeeding teeth are like the blades of shears, 

 adapted to cut and divide soft substances like flesh," &c. Professor 

 Owen has elsewhere described the premolar of Hypsiprymnus as 



* Phil. Trans., vol. cxlix. p. 318, pis. 11 and 13. 



t " The first molar is lunate, the cusps turning inwards, the anterior cusp 

 rising at a salient angle, the edge is trenchant outwards ; the second molar is tri- 

 angular with a large anterior cusp, and a slight ridge passing to a small depressed 

 posterior cusp." — Stutchbury, loc. cit. 



