author of this work (81) have treated of the external form and internal structure of new species of apes, and varieties of the chimpanzee. Much has been written about the orang-utan since Vosmaer’s (82) day, among others by Rademacher (83), Wurmb (84), Griffith (85), Temminck (86), Schlegel and S. Müller (87), Is. Geoffroy Saint-Hilaire (88), Brooke (89), Abel (90), and Wallace (91). Camper (92), Owen (93), J. Müller (94), Schlegel and S. Müller (95), Heusinger (96), Dumortier (97), Brühl (98), Bischoff, Langer (99), etc., have studied the anatomy of this animal. Good illustrations of the orang-utan are found in Vélins’ catalogue, copied by Chenu (100) and Gervais (101), and in Wallace; also in the designs by Mützel (102) and Max (103), and in my work on the Gorilla, already cited. It had been already shown by Tilesius (104) and Cuvier (105) that Wurmb’s young pongo is identical with the orang of Linnæus. We now know certainly that the name pongo (n’pungu in Loango) should only be applied to the gorilla. The fourth and smallest species of anthropoid apes, the Indian long-armed apes or gibbons, have been recently described, with reference to their form and mode of life, by various travellers and naturalists, especially by Duvaucel (106), Bennet (107), Martin (108), Lewis (109), S. Müller (110), Diard (111); also by Buffon (112), Is. Geoffrey Saint-Hilaire (113), and Blyth (114), etc. Gulliver (115), Bischoff (116), and the author of this work have studied the anatomy of these creatures. CHAPTER II. THE EXTERNAL FORM OF ANTHROPOID APES. In the gorilla, the chimpanzee, and the orang-utan the external form is subject to essential modifications, according to the age and sex. The difference between the sexes is most strongly marked in the gorilla, and these differences are least apparent in the gibbon.