Rays and skates are members of the Chondrichthyes, a class of animals that first abundantly populated the oceans during the middle of the Devonian period and has persisted into the modern age. Other members of the class include sharks, chimaeras, and sawfish; sharks representing the best known members of the group. The rays that we are interested in this week are some of the most well known fossil chondrichthyians from the Eocene known as Heliobatis radians (Sun skate/ray). The ray Heliobatis was originally described in 1877 by O. C. Marsh (simply as Heliobatis) and has since had four genera synonymized under the name Heliobatis. Particularly well-known from the Green River Formation, Heliobatis is best known specifically from the Fossil Lake sediments of Wyoming; many of these are housed or on display in either Fossil Butte National Monument ("America's Stone Aquarium") or the Yale Peabody Museum. The known fossils housed and displayed in those two locations are highly detailed. The details are so well defined actually the sexually dimorphic characters, defensive characters, and the feeding apparatus have all been described. Teeth are observable in many of the specimens and based on the triangular shapes of the teeth, the diet that has been hypothesized for Heliobatis is largely based on small fish, crustaceans (there are numerous crayfish and prawn fossils associated with the same formations as Heliobatis), and mollusks.