418 



THE EYE IN EVOLUTION 



Owl -parrot, 

 Stringops 



(3) Temporalmonofoveal. Owls (including the owl-parrot, Strin- 

 gops) have a round macular area in the temporal quadrant with a 

 shallow fovea (occasionally absent). The swift, Micropus, has in 

 addition a trace of a central macula. 



(4) Infula-mo7iofoveal. Some ground-feeders and water-birds, 

 including swimmers, divers and waders, have a central round macular 

 area with a fovea of medium dejDtli tlu'ough which runs a horizontal 

 band of acute vision. These include the albatross, Diomedea cauta, and 

 the giant ]3etrel, Macromectes giganteus (O'Day, 1940) (Plate XII, 

 Fig. 6). 





At'. 



'•■•-t»i» ••♦•» 





Albatross, 

 Diomedea 



Fig. 523. — The Retina of the Albatross, Diomedea. 



Section through the region of the central streak. 1, optic nerve fibre 

 layer ; 2, ganglion cells ; 3, inner plexiform laj'er ; 4, inner nuclear layer ; 

 .T, outer plexiform layer ; 6, outer nuclear layer ; 7, external limiting mem- 

 brane ; 8, visual cells ; 9, pigment epithelium (O'Day). 



(5) Bifoveal. Many birds which seek their prey on the wing 

 (j)asserines, kingfishers, bitterns, humming birds, Calypte, and so on) 

 are commonly jDrovided with a deej^ly excavated principal central 

 fovea and a subsidiary shallower temi^oral fovea surrounded by a 

 smaller macular area lying about the same distance from the optic 

 disc as the central fovea. 



((^) Infnla-bifoveal. Certain predators have two foveae associated 

 with a band of clear vision, (a) The more common arrangement is 

 two circular macule connected by a band, as occurs in hawks, eagles 

 ' '^ swallows ; each macula has a fovea, the central being deepest 

 €' ot in the eagles wherein the temporal is deepest, {b) Alternatively, 



