308 WILLIAM A. HAS WELL. 



jaws. When this part of the apparatus is brought into use 

 the fulcrum is pushed forwards by the action of its protractor 

 muscles, and the two sets of rami, becoming thrust out 

 through the aperture of the moutli, become widely divari- 

 cated with the teeth at their outer ends. When the upper 

 jaw has been fully exserted in this way the right and left 

 rami are brought together sharply, the fulcrum being jerked 

 back slightly at the same time. This series of movements is 

 effected with great rapidity, so that it is extremely difficult 

 to follow and analyse them. The chief part of each of the 

 two lower jaws is a stout, slightly curved rod (/.-), thickest 

 behind and tapering slightly in front. These rods lie nearly 

 parallel with one another, but diverging slightly anteriorly. 

 The most anterior part of each (about one seventh of the 

 length of the whole) articulates with the rest by a transverse 

 joint. Behind this to about the middle of the posterior portion 

 of the rod runs a thin, internally projecting flange with a 

 smooth inner edge. Firmly fixed to the anterior portion of 

 each rod along its inner edge is a broad plate, the inner edge 

 of which is iu contact with that of its fellow of the opposite 

 side. The anterior edge of this plate is finely denticulated ; 

 its antero-lateral angle is produced into a pointed process 

 directed outwards and backwards. 



Connecting together the rami of the upper jaw with the 

 lower jaws on each side is a sort of bridle {hr.) composed of 

 two pieces, the posterior of which curves over the rod of the 

 lower jaw, and slides along it when the upper jaw is pro- 

 truded or retracted. The effect of this arrangement is to 

 restrict the forward movement of the upper jaw, the curved 

 piece being checked when it reaches the anterior broad plate 

 of the lower jaw above described, and further movement of 

 the upper jaw forwards being thus prevented. At tlie same 

 time, as will be explained presently, it is through the inter- 

 mediation of the bridle that the biting movements are carried 

 on. Whether the lower jaws have any function beyond merely 

 supporting the upper jaw and controlling its movements is 

 not clear. I have not observed them performing any iude- 



