 In Proetida and Phacopina from western Europe and particularly Tropidocoryphinae from France (where there is good stratigraphic control), there are well studied trends showing progressive eye reduction between closely related species that eventually leads to blindness.Several other structures on trilobites have been explained as photo-receptors.Of particular interest are "macula", the small areas of thinned cuticle on the underside of the hypostome.