164 ^'I--\V VORK ZOOLOGICAL SOCIETY. 



owing to the loss of the neurilemma, or fatty outer sheath. The 

 fovea must be very indistinct, for in a three hours' examination 

 of six or eight owls no trace of it could be made out. 



Contrary to the rule in the majority of birds, the upper, not the 

 lower, eyelid functions chiefly in the closing of the eye, resem- 

 bling mankind in this respect. The presence of prominent hair- 

 like e3-e-lashes is another feature which aids in giving to some 

 owls such a ludicrously human expression. The third eye-lid or 

 nictitating membrane, is well developed, and is frequently drawn 

 across the eye in the day-time, serving to shut out the blinding 

 glare of the sunshine. 



Yellow predominates as the color of the iris, all of the North 

 American species of owls having this color, except the barn and 

 barred owls, in which the eye is brownish-black, rendering the 

 dark pupil-hole indistinguishable except at close range. 



Ow'ls in searching for their food in the dusk, fly silently over 

 the fields, watching and listening for the slightest movement of 

 the mice beneath them, and this concentration in one direction is 

 most interestingly correlated with the position of the eyes. These 

 are directed forward to a greater degree than in any other group 

 of birds although the facial disks make the convergence appear 

 even greater than it actually is. For example, in barn owls the 

 eyes seem to have almost parallel or convergent vision, as in the 

 higher apes and in man, whereas the eyes of this species have but 

 slightly less divergence than in the great horned owls. In the 

 latter the yellow irides render the detection of divergence from 

 parallel vision more easy to the casual observer.- 



Rough measurements with a goniometer show a divergence of 

 the optical axes in owds (in six living species which I have exam- 

 ined) of from 23 to 36 degrees. This places owls between the 

 evolves and dogs, {Canidac) and the horses, {Equidac) of the 

 mammalia, and gives them about the same divergence as the bears, 

 (Ursidac). Unlike all of these animals, however, the eyeball in 

 owls admits of little or no motion, rotation or otherwise, and 

 hence we have an explanation of the constant movement of the 

 head in these birds, when looking intently about them. 



The circle of bony plates in the sclerotic coat of the eye is so 

 remarkably developed — each plate being so large and the whole 

 fitting so closely together, that the owl is forced to turn its whole 

 head in the exact direction in which it wishes to look. This im- 

 mobility of the eyeball is in ])art compensated for by the unusual 

 amount of play between the fourteen bones of the neck, far ex- 



