Holochroal eyes are compound eyes with many tiny lenses (sometimes more than 15,000, each 30-100μm, rarely larger). They are the oldest and most widespread type of trilobite eye, and found in all orders of trilobite from the Cambrian to the Permian periods. Lenses show hexagonal close packing, and a single corneal membrane covered all lenses. Unlike most modern arthropods, these eyes were not covered by the white layer known as the sclera.
^Clarkson, E. N. K. (1979), "The Visual System of Trilobites.", Palaeontology22: 1–22
^Schoenemann, B. (October 2007), "Trilobite Eyes and a New Type of Neural Superposition Eye in an Ancient System", Palaeontographica Abt. A (Verlag) 281 (1–3): 63–91
^Clarkson, E.N. (1997), "The Eye, Morphology, Function and Evolution", in Kaesler, R.L. (ed), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida, Boulder, CO & Lawrence, KA: The Geological Society of America, Inc. & The University of Kansas, pp. 114–132, ISBN0-8137-3115-1