IT is well known to students of ornithology that a number of birds, particularly such larger forms as eagles, hawks, vultures and owls, have in the sclerotic coat of the eye a supporting ring of bony plates. Less well known is the fact that such a ring is developed to a greater or less degree in the eyes of all species of birds. Even in hummingbirds sclerotic rings are found, made up of minute osseous plates of almost the thinness of tissue paper. Comparatively little attention has been devoted to these rings, although according to Lemmrich (1931), who gives a history and rather complete bibliography of the subject, knowledge of their occurrence in birds of prey dates back to the thirteenth century, and a clear description of them was given by Volcher Coiter in 1645. Students of the osteology of birds, however, have as a rule merely referred to the sclerotic ring in passing, sometimes taking casual notice of the number of plates but giving no consideration to details of shape or arrangement. Slonaker (1918, 1921) has given a careful description of the structure and embryonic development of the scleral plates in the eye of the English Sparrow. Heilmann (1927) has mentioned and figured the scleral ring in certain recent and fossil birds and reptiles. It remained for Lemmrich (loc. cit.) to point out, after examination of representatives of a considerable number of orders and families of birds, that a great deal of variation in the character of the sclerotic ring occurs in different groups, but that a degree of constancy usually prevails within given families or orders. The present study was largely completed before Lemmrich's investigations came to our attention. The findings here reported, however, are in the nature of an extension rather than a duplication of that author's work, inasmuch as he dealt largely with European birds, and in cases where American species were considered his material was rather limited. In the present investigation 1404 pairs of sclerotic rings have been examined in considerable detail, including those of 235 species involving representatives of all of the orders of North American birds except one (no specimens of trogons having come to hand). In general we have several species represented in each order and in a few cases (English Sparrow, Mallard and Glaucous-winged Gull) enough individuals of one species have been available to permit a statistical study.