Retinal twin cones or retinal double cones in fish: Misnomer or different morphological forms?

Scanning retinal electronmicroscopy studies demonstrated the presence of both twin cones (two identical cones adhered together) and double, cones, (two nonidentical cones adhered together) in the retinae of the major forms of fish, which included actinopterygi, elasmobranch, and telcosts. The complexity of these visual cells in the retinae of different groups of fishes was revealed for the first time and reflected on the diversities of functions of the fish retina it) general.