The eyes of Asaphus raniceps Dalman (Trilobita)

The holochroal eyes of the Lower Ordovician trilobite Asaphus raniceps Dalman have been studied using light and electron microscopy.In these eyes the refractive elements are elongated calcite prisms underlying a cornea which is continuous with, although structurally dissimilar to, the 'outer cuticular layer' described by Dalingwater. The prisms are orientated with their c-axes normal to the surface. Some of the material studied (from Oland) showed the effects of at least two phases of diagenesis, which in one case had resulted in the production of secondary prisms growing syntaxially on the primary prisms, and confusingly similar to primary structures.The visual surface approximates a segment of an almost perfect spheroid whose radii of curvature in vertical and horizontal planes all converge to a single point, in a manner very similar to that of some superposition eyes in modern arthropods, with which analogies are drawn.Problems in the use of calcite as a primary refractive medium are discussed, and it is concluded that the effects of birefringence could have been minimized by suitable pigment screens, like those of insects and crustaceans, underlying the prismatic layer.The 'sensory fossettes' on the eye-socle are craters, each with a central perforation communicating with the internal surface.