_Ateles hypoxanthus_, Desm., Mamm., p. 75 (1820); Neuwied, apud Kuhl, Beitr. Zool., p. 25 (1820); Schl., t. c. p. 185 (1876, part.). _Brachyteles macrotarsus_, Spix, Sim. et Vespert., Bras., p. 36, pl. 17 (1823). _Eriodes hemidactylus_ and _E. tuberifer_, Geoffr., Mém. Mus., xvii., pp. 161, 163 (1828). _Eriodes arachnoides_, Geoffr., Mém. Mus., t. c. p. 160 (1828). _Brachyteles arachnoides_, Gray, Cat. Monkeys Brit. Mus., p. 45 (1870). CHARACTERS.--MALE.--Size small; face nude, flesh-coloured; general colour of body yellowish-brown, darker on the back of the head, with a few long black hairs on the forehead; hairs of head short and directed backward; buttocks, vent, base of tail and perineal region dark ferruginous-brown; the thumb wanting or rudimentary. Length of body, 22 inches; tail, 26 inches. FEMALE.--Ashy-brown, instead of yellowish-brown, in appearance. YOUNG.--In some young specimens the general colour is dark brown, with the sides of the face white. Dr. Slack observes, in the "Proceedings of the Academy of Natural Sciences of Philadelphia" for 1862, in reference to this species: "I had long suspected that the three species of this genus described by Isidore Geoffroy St. Hilaire, were in reality one and the same; no specific characters are manifest in their coloration, or skulls, the supposed differences being {227}based upon the development of the anterior thumbs, this member being absent in _B. arachnoides_, replaced by a small nailless tubercle in _B. tuberifer_, and surmounted by a nail in _B. hemidactylus_. In the "Magazin" of Messrs. Verreaux, in Paris, I found specimens having upon one hand the tubercle, and upon the other the nailed thumb, others with the tubercle on one hand, but absent upon the other. St. Hilaire himself, in his "Catalogue of the Primates," expresses a doubt as to whether _B. arachnoides_ and _B. hemidactylus_ are really distinct. In September and October, 1860, I was unable to find _B. hemidactylus_ in the Paris Museum, all the _Brachyteles_ being labelled _Eriodes arachnoides_." DISTRIBUTION.--Confined to the wooded region of the south-east of Brazil. HABITS.--Arboreal, diurnal, and (it is supposed) gregarious, frequenting the high forest trees, and subsisting on fruits. THE SPIDER-MONKEYS. GENUS ATELES. _Ateles_, Geoffr., Ann. Mus., vii., p. 262 (1806).