Article: Schizochroal eyes and vision in some phacopid trilobites

Some aspects of vision in nine species of phacopid trilobites are described. Methods of study, which primarily consist of the investigation of the morphology of the visual surface and the extent and nature of the visual field, are identical with those of previous work.Divergent interpretations of intraspecific variation in the lens number of Phacops sp. are discussed and sexual dimorphism is advanced as a tenable hypothesis. Phacopids generally have a fixed pattern of lens distribution, though the individual lens formation and arrangement varies in different species.Individuals within a species have visual fields of similar extent and type, regardless of dimorphism. In different species, however, there is wide variation in visual type, largely as a result of distinct modes of curvature of the visual surface, affecting the angular bearings of the lens-axes.Three principal visual types are distinguished, presumably reflecting adaptations to distinct ecological conditions. In the first, the visual field has a relatively wide vertical range and the eye is slightly astigmatic; in the second there is much greater astigmatism and a narrower visual field; the third type combines features of the other two.