IGO 



Now let us return to the application of the Aye-aye's mandibular structure to the 

 explanation of that in Thylacoleo and Plagiaulax. "The large front teeth in Chiromys 

 are curved in segments of circles, the working surface is elongate, in breadth equalling 

 that of the base of the tooth, with a front convex enamelled border, forming the obtuse 

 apex of the gouging surface"*. 



With what molars are these scalpriform teeth associated \ Few, small, tubercular. 

 Adapted for squeezing the soft animal nutriment out of the tegumentary covering of a 

 caterpillar, not adapted for trituratory mastication of such vegetable food as calls for 

 the more complex and massive molars of the Kangaroos, Potoroos, and Koalas. With 

 what kind of teeth is the low-placed and backwardly placed condyle of Thylacoleo and 

 Plagiaulax associated 1 and what may be the diet indicated by such association \ For 

 the response to these questions the palaeontologist, guided by the Cuvierian principle, 

 refers to the great carnassial and the small tubercular molar teeth. 



Comparison of the Mandibular Condyle in Thylacoleo, Plagiaulax, and Bodentia. 

 — In placental Rodents the mandibular condyle is longitudinally horizontal, transversely 

 convex ; its long diameter is from before backward ; it represents the section of a cylinder. 

 The glenoid cavity of Thylacoleo shows that its condyle has been convex from before 

 backward or longitudinally, and with its long diameter transverse; not limiting the 

 jaw, as in Rodents, to horizontal movements chiefly to and fro, but adapting the jaw to 

 hinge-like vertical motion, needed for the due action of the terminal laniaries and the 

 large carnassials. 



The mandibular condyle in the more ancient and smaller Marsupials with a closely 

 analogous dentition is demonstrably similar to that which is here inferentially ascribed 

 to the condyle of Thylacoleo. It is in Plagiaulax convex longitudinally, or from before 

 backward, and that in so great a degree that the most prominent part of the convexity 

 looks backward. " Its long diameter is disposed subvertically, and the outline is ovate or 

 pyriform, the broad end being uppermost "f. This broad end is the transversely ex- 

 tended part of the convex condylej. Dr. Falconer, nevertheless, affirms that the form 

 of condyle presented by Plagiaulax is " common in the placental Rodents "§ ; yet is con- 

 strained to add, " with the difference, however, that in the latter the condyle having to 

 work backwards and forwards in a groove, its articular surface is disposed longitudi- 

 nally" || . 



But this difference precludes an ascription of community of form of condyle between 

 Plagiaulax and Rodentia ; and in so far as the difference is such as to enable the con. 

 dyle in Plagiaulax to work the jaw upward and downward, or vertically, it lends itself 

 to those actions which the jaw has to perform " among the Carnivora? 



The kind and degree of difference which the mandibular condyle presents in Chiromys 

 and in Plagiaulax, already pointed out, in like manner illustrates its application in the 



* Owen, ' On the Aye-aye,' 4to, 1863, p. 25. t X. p. 3G0 ; XI. p. 445. 



% Owen, Monograph on British Mesozoic Mammalia, pi. 4. fig. 10, A, c & B. 

 § X. p. 360 ; XI. p. 445. || Id. ib. 



