164 NEW YORK 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 eye-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. 
Owls in searching for their food in the dusk, fly silently over 
the fields, watching and listening for the slightest movement oi 
the mice beneath them, and this concentration in one direction is 
most interestingly correlated with the position of the eyes. These 
ave 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 owls (in six living species which I have exam- 
ined) of from 23 to 36 degrees. This places owls between the 
wolves and dogs, (Canidae) and the horses, (Equidae) of the 
mammalia, and gives them about the same divergence as the bears, 
(Ursidae). 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 1m- 
mobility of the eyeball is in part compensated for by the unusual 
amount of play between the fourteen bones of the neck, far ex- 
