paratively flat, and the lens almost globular in 

 fishes, in birds the cornea is remarkably promi- 

 nent, and the lens has very little convexity. The 

 motions of the iris in most birds are extremely 

 rapid, and in some apparently voluntary. The 

 pupil is in some, as the dove and the goose, 

 transversely oval, while it is vertically oval in 

 others, as the owl : generally speaking, indeed, 

 it has the former shape in herbivorous animals, 

 whether birds or quadrupeds, and the latter in 

 carnivorous. All birds have proper eye-lids, the 

 lower of which alone is moveable ; and they 

 have, in addition, another membrane called 

 membrana nictitans, which is merely a moveable 

 fold of the external membrane of the eye-ball : it 

 is not quite proper to birds being found also in 

 some fishes and reptiles but it is most remark- 

 able in them. With very few exceptions the 

 owl among others the direction of the eye-balls 

 is, in birds, outwards. Such birds also, as well 

 as insects and fishes, as go in search of their 

 prey by night, like the owl, have a shining sub- 

 stance at the bottom of the eye-ball, for the pur- 

 pose already alluded to. In some birds with 

 piercing sight, as the falcon and crane, the flat- 

 tened optic nerve has one of its surfaces folded 

 into numerous plaits, bearing the same relation to 

 to the other as the leaves bear to the back of a 

 book ; and the extent of surface thus gained may 

 be easily imagined. 



Among the mammiferous animals, the cetace- 

 ous tribes, as we should expect from their habits, 



