Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Double cones (DCs), also known as twin cones in some literature, are two cone cells (colour detecting photoreceptors) joined together that may also be coupled optically/electrically They are the most common type of cone cells in fish, reptiles and birds, and are present in most vertebrates, though they have been noted as absent in placental mammals, elasmobranches and catfish. There are many gap junctions between the cells of fish double cones.