458 SLONAKER. [Vol. XIII. 



of the eye in the head. As the eyes are turned more and more 

 forward, the fovea temporalis approaches the fovea nasalis, and 

 as binocular vision becomes more frequent, the nasal fovea 

 becomes less distinct or merges into the temporal. This change 

 is shown in Plate XXXI. The change to an asymmetrical 

 form, and the position of the lens in the eyes of the birds which 

 possess the power of binocular vision, is also quite marked. In 

 many of these birds, as the tern and white-bellied swallow 

 (PI. XXIX, Figs. 40, 41, 46, and PI. XXXI, Figs. 64, 65), 

 where the fovea nasalis is sharp and deep and the fovea tem- 

 poralis quite shallow, the eyes are almost symmetrical. But 

 in those birds which use binocular vision more as shown by a 

 greater depth of the temporal fovea, as in the hawks (PI. 

 XXIX, Figs. 48-51, and PL XXXI, Figs. 66, 67), the eye 

 becomes more asymmetrical, and finally reaches the most 

 irregular form in the owl (PI. XXXI, Fig. 6S), where binocular 

 vision only occurs. 



Various combinations of area and fovea are found in different 

 animals. The most simple is a single fovea surrounded by a 

 circular area, as in the primates and most birds. Again we 

 find a simple fovea surrounded by a round area which is con- 

 tinuous with a band-like area extending horizontally across the 

 retina, as in the goose and ring-neck plover. One or two foveas 

 may be present, each surrounded by a round area and connected 

 by a short, slight, band-like area, as in the sparrow hawk, red- 

 tailed buzzard, and kingfisher. The most complex combination 

 which I have found is in the tern. Here the fovea temporalis 

 surrounded by a small round area is not connected with the 

 band-like area which extends horizontally across the retina, and 

 near its middle widens out into a round area surrounding the 

 fovea nasalis (PI. XXVII, Fig. 12). 



In my researches I have been able to examine 93 different 

 species, of which 18 were mammals, 41 birds, 6 reptiles, 3 am- 

 phibians, and 25 fishes. In some cases the results were doubt- 

 ful, and sufficient material was not available to ascertain all 

 points with certainty. Such cases I have indicated. Many of 

 the species examined have been observed by others, in which 

 case I have always aimed to give credit to the first observer. 



