Mr. Yarrell on the Beak of the Crossbill. 461 



letter c,) affording firm support to the roovecible portion of the upper 

 mandible. Letters d. d. figure 3, refer to the jugal bone, which, united 

 to the superior maxillary bone in front, is firmly attached by its posterior 

 extremity to the outer side of the os quadratum ; when, therefore, the 

 OS quadratum is pulled upwards and forwards by its own peculiar mus- 

 cles, to be hereafter mentioned, the jugal bone on each side by its 

 pressure forwards elevates the upper mandible. 



The inferior projecting process of the os quadratum, to which the lower 

 jaw is articulated, in most other birds is somewhat linear from before 

 backwards, and compressed at the sides, admitting vertical motion only 

 upwards and downwards ; the same processes in the Crossbill are sphe- 

 rical, as shewn at c. figure 3 ; the cavity in the lower jaw destined to 

 receive this process is a hollow circular cup (figure 5, letter a) ; the union 

 of these two portions therefore forms an articulation possessing the uni- 

 versal motion and flexibility of the mechanical ball and socket joint. 



The lower jav/ is of great strength, the sides or plates elevated, with 

 prominent coronoid processes, (figure 5, h. 6.,) to which, as well as to 

 the whole outer side 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 extent I have already mentioned of ^ of 

 an inch. 



The temporal and pyiamidal 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, as represented in figures 1, 2 and 4, letters 

 a. and h., and indicated by their bulk the great lateral power this bird is 

 capable 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 extremities of the lower jaw were removed, and 

 the food of the bird being small seeds rendered a narrow pharynx suffi- 

 cient for the purpose of sw^allowing. 



The muscles depressing the lower mandible are three in number, only 

 one of which, the great pyramidal, is visible, figures 1, 2 and 4. letter h. 



