Descriptioii of the Modifications of certiiin Organs. 631 



exaiiimation of a coiisiderable Duniber of speciiiiens in tlie Collection 

 of the University of Cambridge, together with the observations made 

 by TftWNSON and Yaruell, yields tlie followiiig results. 



Both, the Upper and lower halves of the beak are drawn out into 

 eng, pointed and curved hooks and cross each other, the niandible 

 Ibeing curved upwards , the premaxilla downwards. This crossing 

 occurs in all the species of the Genus Loxia ; this genus has a wide 

 distribution , Crossbills inhabiting the pineforests of North-Anierica, 

 the Northern half of the palaearctic region, and the Hinialayas. 



The mode of crossing varies individually in every species, i. e. 

 the niandible is turned either to the right or to the left, no predi- 

 lection for one side being shown. Out of 36 specimens, belonging to 

 six species examined, 20 had the niandible turned towards the right, 

 IG to the left side. Of the 11) niales 12 weie rightbilled and 7 left- 

 billed, whilst of the 13 feniales 6 were right and 7 leftbilled. Of the 

 reniaining 4 young birds the sex was undeterniined. 



The amount of curvature exhibited by the upper and lower beak 

 varies considerably. (See figures 1 — 3). As a rule the lateral dis- 

 tortion is niore niarked in the niandible, but in fully adult birds nearly 

 the whole of the upper beak is also laterally distorted. Where the 

 cutting edge of the left niandible meets the right edge of the pre- 

 maxilla, or vice versa , these edges are turned inwards to -a varying 

 extent, but are not filed away, so as to enable the beak to be closed ; 

 sonietimes however a gap remains. 



As Yarrell ' ) has shown, this distortion and the peculiar mode 

 of feeding of the Crossbills produces an asymmetrical arrangement of 

 the whole masticatory apparatus. The temporal and pyramidal muscles 

 on that side of the head to which the lower jaw is inclined, are con- 

 sideralily larger than those of the other side, and indicate the great 

 lateral power these birds are capable of exerting. Moreover not only 

 the muscles Ijut also the bones of the same side are aflfected. In 

 rightbilled specimens the right quadrate is much stronger, and the 

 right half of the occipito-squamosal region is larger than the corre- 

 sponding left parts. The quadrate possesses a large spherical condyle, 

 which fits into a corresponding hollow circular cup of the mandible, 

 tliis arransement admitting therefore the universal niotion of a ball 



li W. Yarrell, ,,0n the structure of the beak and its muscles in 

 the Crossbill {Loxia curvirostrisY' , in: Zoological Journal, vol. 4, 1829, 

 p. 459—465, pl. XIV. 



41* 



