18GG.] ON THE LOWER JAW OF THE KANGAROOS. 33 



while the incisors are asunder, leads to the inference that they like- 

 wise modify and aid the opening movement, although mere position 

 and the parallelism of their fibres to the median line of divergence 

 (at least as they appear on dissection) makes one hesitate to attri- 

 bute too much power to them ; hut they certainly act along with, 

 and greatly strengthen the force of, the mylo-hyoidei. 



By the conjoined simultaneous action of the whole of the 

 muscles mentioned, the movement of the symphysis and separation 

 of the incisors seem to be effected ; while the return to the state of 

 closure follows relaxation of these, with possibly contraction of one 

 of the pterygoid muscles. The chief agent, however, in the approxi- 

 mation of the anterior portions of the symphysis and the internal 

 edges of the incisors is no doubt the transverse fibres of the orbicularis 

 oris (here situated at the anterior portion of the bony symphysis and 

 the root of the lower incisors), which, although delicate medially, is 

 nevertheless well developed laterally (see figs. 1 and 2, b). 



In proof that the portion of the symphysis in close juxtaposition 

 are the pivot or point of leverage in the movement spoken ot, and that 

 the muscles stated are those concerned in the action, a partially 

 dissected specimen need only be experimented on, when very gentle 

 inward pushing at the angle will be seen to produce the separation 

 of the incisors ; and a like lbrce applied in advance of the pivot (e. g. 

 where the orbicular muscle is placed) immediately and easily causes 

 closure. 



Hence, as to the point at issue, we have tried to show from our 

 observations the analogy of the movements and use of the mandible 

 and incisors of the Kangaroo to those of a pair of cutting-forceps or 

 scissors, the posterior part of the symphysis being the pivot, the 

 angles the handles, and the incisors the "blades, the inner edge of 

 which is the cutting-edge. 



The manner of use, at least in confinement, we have attempted to 

 describe ; but whether the teeth are put to the same use when the 

 animal is in a state of nature is a fact unknown to us. Mr. Gould, 

 whose opportunities of observation of their native habits was at one 

 time great, informs us that the food of the Petrogale brachr/otis is 

 often dry and tough vegetables obtained among rocky places. It 

 is possible therefore that the cutting-edges in that case might well 

 serve to sever dry or fibrous material. 



P.S. Since this paper was read, our attention has been called by 

 Dr. J. E. Gray to a paragraph in 'The Book of Nature,' by John 

 Mason Good (vol. i. p. 254), where the author says, "The Mus ma- 

 ritimus, or African Rat, the largest species of this genus which has 

 hitherto been discovered, and seldom less than a full-sized rabbit, 

 has the singular property of separating at pleasure to a considerable 

 distance the two front teeth of the lower jaw, which are not less than 

 an inch and a quarter long. That elegant and extraordinary crea- 

 ture the Kangaroo, which we may soon hope to see naturalized in 

 our own country, is possessed of a similar faculty." 



But this statement does not seem to forestall the facts which we 

 have observed regarding the use or manner of action of the lower 



Proc. Zool. Soc— 1860, No. III. 



