32 DR. J. MURIE AND MR. A. D. BARTLETT [Jan. 9, 



within the apex of the foremost pair of incisors of the upper jaw, the 

 two sharp inner edges of the lower incisors coming so close together 

 that the line of separation (fig. I, a) is hardly distinguishable; but 

 when the angles of the inferior maxilla are slightly approximated, 

 as, for instance, by a gentle pressure of the thumb and fore finger, 

 then the two teeth open like the blades of a pair of scissors at their 

 points, as much as one-eighth of an inch, and, according to what 

 Mr. Waterhouse says, their outer cutting-edges are brought more 

 closely in contact with the inner edge of the anterior upper incisors ; 

 but at the same time it must be remarked they can also overlap them 

 (fig. 2, a). But whether this overlapping takes place ordinarily, 

 when the creature crops its food, we are not prepared to say, as it 

 is not readily distinguishable, from the rapidity of the act. 



In the second specimen of Kangaroo (Petrogale brachyotis), an 

 adult female, the tips of the lower incisors could be separated almost 

 one-fourth of an inch, and the structure was in nearly all respects 

 similar to that of Bennett's Kangaroo — with this difference, that in 

 Petroyale brachyotis there was an interspace at the base of the lower 

 incisors, even when the points of the teeth were brought in contact ; 

 while in Halmaturus bennettii the whole of the inner edges were 

 applied closely to each other (fig. I, a). It is possible that this slight 

 hollow may have been caused by a wearing away of the substance ; 

 that it existed in the adult animal gives feasibility to this belief. 



The next point of interest connected with this remarkable move- 

 ment is the consideration of what muscular apparatus or set of 

 muscles produces it. 



The great breadth and increased size of the inner hollow of the 

 ramus and angle of the mandible in the Marsupials at once suggests 

 that the pterygoid muscles, from their increased purchase and posi- 

 tion, would entirely effect this, as it does, to a great extent, in the 

 bovine race the process of rumination. No doubt these muscles 

 are concerned in the motive act of the one half of the mandible 

 upon the other in the Kangaroos ; but certain other muscular fibres 

 seem also to be called strongly into play. 



The thin layer of the platysma myoides on either side appears to 

 have a slight influence in the production of the opening of the inci- 

 sors, by gently aiding the approximation of the angles of the lower 

 jaw. 



The digastric muscles, moderately strong in the specimens dis- 

 sected, have their usual origin and median tendon slightly in advance 

 of the angles. Their anterior fleshy bellies are inserted half an inch 

 or so behind the posterior junction of the symphysis, so that on con- 

 traction of their fibres they serve to pull together the posterior rami, 

 and also produce the aforesaid separation of the lower incisors. 



The two mylo-hyoidei are not extraordinarily large, although 

 broad ; but their position and nearly transverse direction give them 

 even a more direct and important action in the opening movement 

 than the last. 



The genio-hyoidei are, on the contrary, strong and well developed. 

 Their point o^ traction from the hyoid bone, and apparent tenseness 



