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external edge of the eye, and certainly no degree of convexity 

 at all approaching to that of the Golden Eagle. The shape of 

 the individual bones is so various that it cannot be given in any 

 general term ; the external edge of the bones is in most 

 instances beautifully serrated ; but the serration is not visible 

 in the bony ring ; but in the separate bones which were boiled 

 until the extraneous matter would wipe off easily with a cloth, 

 it is very perceptible ; the rings would separate if boiled to the 

 same extent, and in cleaning them with a knife I have not 

 been able to preserve the serrations. 



As regards the structure of the rings, the bones generally 

 overlap each other, there being a depression on the under side 

 of one bone, and a precisely corresponding one on the upper 

 side of its neighbour, so that when overlapping each other, they 

 present nearly an even surface, both exteriorly and interiorly ; 

 having one bone with both depressions on its inner surface, and 

 forming an exterior key to the arch, and one having both 

 depressions on its outer surface, and forming an internal key ; 

 in some instances there are two external keys ; and in several 

 instances, instead of one bone overlapping the other, they inter- 

 lock into each other in a curious and beautiful manner. 



I take it for granted that the principal function of these 

 sclerotic rings is, as stated by Cuvier, to preserve that peculiar 

 form of the eye which is adapted to the nature and wants of the 

 animals. But in nature's laboratory there is no prodigality or 

 waste of power ; and these rings having, in the first place, 

 fulfilled the duties for which they were primarily created, still 

 subserve the wants of the animal in other ways, and under cir- 

 cumstances and in situations to which those classes of animals 

 which have not such rings are not exposed. 



I allude to their use as a defence and protection to the eye. 

 On examining these specimens it will be found that those birds 

 which are peculiarly pugnacious ; those which have a peculiarly 

 rapid flight ; and those which, from the extended variation of 

 altitude at which they fly, are exposed to great or very unequal 

 degrees of atmospheric pressure, have the sclerotic rings of 

 larger size, of more convex form, and the individual bones of 



