362 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 4, 



measurements of the relative proportions of the lower jaw in the 

 Aye-Aye and P. BecMesii * : — 



Cheiromys Flag. 



Madagasc. BecMesii. 



inch. inch. 



Length of jaw from condyle to incisive border 2-3 2-0 



From condyle to posterior edge of coronoid -6 '5 



Height of jaw to summit of coronoid 1*2 1*0 



Height of ramus in front of first true molar -7 *6 



Height of ramus behind the incisor -65 "45 



Height from condyle to aline dropped vertically behind 



lastmolar 1-25 1'05 



Height from the latter point to posterior edge of incisor 



at diasteme ^ "8 '75 



From these proportions it will be seen that both in Cheiromys and 

 Plagiaulax the condyle projects behind the edge of the coronoid to 

 the excessive extent of about one-fourth of the entire length of the 

 ramus. Professor Owen meets the argument in my paper, by the 

 assertion that the condyle of Plagiaulax is " pedunculate as in the 

 predaceous marsupials." If so, I invite him to adduce the instance, 

 bearing in mind that the question here is one of degree. The lower 

 jaw of a Tiger now before me measures 9-2 inches from the condyle 

 to the incisive border, while the projection of the articular surface 

 behind the fall of the coronoid does not exceed -7 of an inch, or one- 

 thirteenth of the length of the jaw. In Dasyurus and Thylacinus f 

 the condyle projects behind the coronoid, but nothing approaching 

 the extent seen in the Aye-Aye and Plagiaulax. 



As regards the functional effect of the condyle being carried so far 

 back behind the edge of the coronoid, it is a plain question of animal 

 mechanics, which the author of the ' Palaeontology ' thus interprets : 

 "It is pedunculate, as in the predaceous Marsupialia, whilst the 

 lever of the coronoid process is made stronger by the condyle being 

 carried further back than in any known carnivorous or herbivorous 

 animal." As I regard it, a necessary effect would be to restrict the 

 power of separating the jaws in front, essential to a predaceous 

 animal having laniary teeth constructed to pierce, retain, and kill. 

 And we have the direct proof in the Aye-Aye, that the same 

 arrangement there is not applied to a carnivorous function :j:. 



* It must be borne in mind that fig. 1 of my previous communication (op. cit. 

 p. 278), from which the measurements of P. BecMesii are taken, is magni- 

 fied two diameters; the dimensions are therefore doubled. But this does not 

 interfere with the r?^tios of proportion. Further, in the Aye-Aye the posterior 

 mai*gin of the coronoid is assumed to be continued down vertically, in order to 

 get corresponding measurements. The dimensions of Cheiromys are of the 

 natural size. 



T In the Ursine Dasyurus (No. 1900, Mus. R. Coll. of Surgeons) the length 

 of the lower jaw is 4-2 inches, and the projection of the articular surface behind 

 the deepest part of the sigmoid notch -4 inch., or about one-tenth of the entire 

 length of the jaw. In Thylacinus (No. 1903 a of the same collection) the pro- 

 jection of the condyle is about one-eighth the length of the jaw. But in both 

 these forms the posterior edge of the apex of the coronoid overhangs the condyle ; 

 while both in Vl. BecMesii and the Aye-Aye the articular surface of the condyle is 

 removed about one-fourth of the length of the jaw behind the fall of the coronoid. 



X In the typical Carnivora the fulcrum is a fixed point, the form of the glenoid 



