COMMON CROSSBILL. 29 



The lower jaw is of great strength, the sides or plates ele- 

 vated, with prominent coronoid processes, figure 5, i, Z>, to 

 which, as well as to the whole outer surface of the plates, the 

 temporal muscle is attached ; and in a head of this bird, 

 which had been divested of all the soft parts, I found, on 

 sliding the lower jaw laterally upon the upper, as performed 

 by the bird, that before the coronoid process is brought into 

 contact with the pterygoid on its own side, the extreme 

 points of the mandibles were separated laterally to the ex- 

 tent I have already mentioned, — namely, three-eighths of an 

 inch. 



The temporal and pyramidal muscles on the right side of 

 the head, that being the side to which the lower jaw inclined, 

 were considerably larger than those on the left side, as re- 

 presented in figures 1, 2, and 4, letters a and b, and indi- 

 cated by their bulk the great lateral power this bird is capa- 

 ble of exerting, to be hereafter noticed. The unusually 

 large size of the pterygoid muscles on each side was very 

 conspicuous, figure 2, letters c, c ; the space for them being 

 obtained by the great distance to which the articulated ex- 

 tremities of the lower jaw were removed, and the food of the 

 bird being small seeds, rendered a narrow pharynx sufficient 

 for the purpose of swallowing. 



The muscles which depress the lower mandible are three 

 in number, only one of which, the great pyramidal, is visible, 

 figures 1 and 4, letter b. This large and strong muscle 

 covers two other small ones, the triangular and square mus- 

 cles, so called from their peculiar shape. These three 

 muscles, all of which have their origin on the occipital por- 

 tion of the cranium, are inserted by strong tendons on the 

 under and back part of each extremity of the lower jaw, 

 behind the centre of motion, and consequently by their si- 

 multaneous contraction raise the point to which they are 

 attached, and depress the anterior part of the mandible. 



