ARE^ CENTRALES AND FOVEJE 



189 



in some ancestor, tended more toward the convexiclivate type of profile. 

 The visual cells of Sphenodon show that this animal was once diurnal 

 (see Chapter 16, section C) and at that time it no doubt had a fovea 



Fig. 81 — Central (nasal) fovea of the European bank swallow. 



Exemplifying the deep, convexiclivate type characteristic of birds and lizards. After Rochon- 

 Duvigneaud. 



Fig. 82 — Fovea and surroundings in Sphenodon. x 90. 



Illustrating the shallow, concaviciivate type characteristic of fishes and of those vertebrates 

 whose fovece have become degraded through domestication or the abandonment of strict 

 diurnality. s, sclera; c, chorioid; r, retina. (The retinal and chorioidal pigment have been 

 bleached from the section; note that only rods are present — this is the only rod fovea in a 

 terrestrial vertebrate). 



