SEM picture of portion of the visual surface of Paralejurus brongniarti (Barrande) (xl08). Devonian, Bohemia. (Negative loaned by E. N. K. Clarkson.)

have formed sharp primary images of a portion of the trilobite's surroundings about 0.2 millimeters below the inner vertex of each lens (a distance equal to the thickness of the lens). This presents a puzzle. For if the photoreceptors were this close to the surface of the eye, where they could make use of primary images, why was the overall structure of the eye so deep—as if it had been constructed to accommodate long rhabdomes? It may be that the photoreceptors were in fact placed further back in the eye than the evidence we have would suggest. Certainly, the possibility that collective diffraction phenomena might result from the array of primary images would have been a good reason for placing the photoreceptors further away from the visual surface. ( This was, in fact, recently discovered to be a relevant factor in the vision of insects, as reviewed by Burtt and Catton 1966.) Even though the principal Fourier image would appear much further behind the eye in this case, intermediate images could still be probed at several locations within the eye cavity, to the trilobite's great advantage. Unfortunately, we have reached at this point an impasse in our investigation. The marvelous optical interface at the surface of the holochroal eye is all that is left to us. We see nothing meaningful beneath it—nothing to either confirm or deny our suppositions about internal structures.

Front (a) and side (b) view of the eye of Pricyclopyge binodosa (Salter) (xl3). Ordovician, Bohemia. (Negative loaned from E. N. K. Clarkson.) In this unusual process of fossilization, only the framework of the visual surface is preserved.

Top view of the eye of anothet specimen of Pricyclopyge binodosa (Salter), as in plate 19 (xl8).

SEM views of the prismatic structure of the visual surface of Asaphus raniceps Dalman, a Lower Ordovician trilobite from Northern Oland, Sweden (a, xl50; b, x33 0). (Negative loaned by E. N. K. Clarkson; Clarkson 1973b.)

(a) Cross-sectional view of the superposition eye of the night moth, in the light-adapted condition (adapted from Hoglund 1965). (b) Cross-sectional view of the holochroal eye of Asaphus (adapted from Clarkson 1973b) (see also plates 22 and 23). The ommatidial arrangement was probably similar in the two cases.

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