Notes on the retinae of two opossum genera

In Didelphis virginiana and Marmosa mexicana the retina is sauropsidan in type and the eye as a whole is markedly adapted for scotopic vision. The superior half of the fundus of Didelphis is occupied by a tapetum lucidum of the retinal type, formed by the enlarged pigment epithelial cells which are devoid of pigment and packed with reflective particles of unknown composition. Such a tapetum is unique among mammals—in all known sub‐mammalian retinal tapeta the reflective material is guanin and is occusible in bright light by migratory fuscin. The retinal capillaries in Didelphis extend to the external limiting membrane, apparently in adaptation to the low permeability of the dense tapetum. Only four other mammals are known to have such capillaries and no inclusive explanation seems to hold for all. Didelphis possesses an ‘area centralis of sensitivity’ associated with the tapetum. Both genera have abundant rods and scanty cones, of three types. The oil droplets of the droplet‐bearing single cones and the double cones are colorless and the double cone contains only one, not two as reported by O'Day for other marsupials; nor are the members of the double cone ever alike in size and form. Opossum double cones are thus perfectly orthodox; primitive placental mammals should be examined for double cones and oil droplets.