Certain females, examined by Dr. Sclater, had a distinct black line along the side of the crown above each ear and extending in front, down the side of the face, nearly to a level {157}with the angle of the mouth; but Mr. Buckley says the Indians consider the black lines to be merely a sign of age. DISTRIBUTION.--This species has an extensive range, being found on both banks of the Amazon, Rio Negro, and on the Copataza river; also in Guiana, Surinam, and Colombia, near Bogotá. HABITS.--Like its congeners, the Common Squirrel-Monkey is arboreal, going about in large flocks. Their food consists of insects and fruits. Mr. Bates observes that the "pretty little _Chrysothrix sciureus_ contents itself with devouring what fruit it can on the spot," thus differing from certain species of _Cebus_, which destroy more than they eat, and when about to return to the forest, carry away all they can in their hands or under their arms. Mr. Darwin has remarked in his "Expression of the Emotions in Man and Animals," that "with Monkeys the expression of any painful emotion is not easily distinguished from that of anger." "Humboldt," he adds, "also asserts that the eyes of the _Callithrix sciureus_ 'instantly fill with tears when it is seized with fear'; but when this pretty little Monkey in the Zoological Gardens was teased, so as to cry out loudly, this did not occur. I do not, however, wish to throw the least doubt on the accuracy of Humboldt's statement." This species is a great favourite wherever it has been kept in captivity. It is very bright coloured, has a baby-like face, large and bright eyes, and most gentle manners. These Monkeys are very sensitive to cold, and when a sudden fall in the temperature takes place, they huddle close together, clasping each other with their arms, and embracing their neighbours and themselves with their long tails. {158}IV. THE RED-BACKED TITI. CHRYSOTHRIX OERSTEDI. _Saimaris sciurea ?_ (nec Linn.), Sclater, P. Z. S., 1856, p. 139. _Chrysothrix sciurea_ (nec Linn.), Scl., N. H. Rev., 1861, p. 510; Frantz. Arch. f. Naturg., xxxv. (1), p. 260. _Chrysothrix oerstedi_, Reinh. Vidensk. Medd. Nat. For. Kjöbenh., p. 157, pl. iii. (1872); Alston, in Godm. et Salv., Biol. Centr. Am. Mamm., p. 16, pl. ii. (1879). _Saimiris entomophaga_, Sclater, P. Z. S., 1872, p. 3 (nec d'Orb.). _Saimiri örstedii_, Schl., Mus. Pays Bas, vii., p. 245 (1876). CHARACTERS.--Differs from _C. sciurea_ in having the top of the head black, the back and sides shining red, and the limbs olive. DISTRIBUTION.--Panama, Guatemala, Costa Rica, especially their hotter districts,--being particularly abundant in the Valley of Terraba and on the plain of Piris. THE TITI MONKEYS. GENUS CALLITHRIX. _Callithrix_, Geoffr., Ann. Mus., xix., p. 112 (1812).