Among vertebrates which are not too strongly nocturnal a local modification of the retina for maximal visual acuity is of widespread occurrence. The modified area (which is single except in some birds) may be small or extensive, a broad horizontal band across the fundus or a suprapapillar crescent, round or in the shape of a variously oriented oval, located centrally or temporally even as far as the periphery and on or slightly above or below the horizontal meridian. The generic term for such a region of high resolving power is area centralis—misnomer though this name often is. An area centralis may be distinguished from the surrounding retina only by the visual cells being more numerous in it per unit of retinal area. When this massing of visual cells is marked, however, it is usual to find several accompanying adaptations: The visual cells are slenderized, to facilitate their aggregation; they