“Young Orangs when tickled likewise grin and make a chuckling sound, and Mr. Martin says that their eyes grow brighter. As soon as their laughter ceases, an expression may be detected passing over their faces, which, as Mr. Wallace remarked, may be called a smile. I have also noticed something of the same kind with the Chimpanzee. Dr. Duchenne--and I cannot quote a better authority--informs me that he kept a very tame Monkey in his house for a year, and when he gave it during meal times some choice delicacy, he observed that the corners of its mouth were slightly raised; thus an expression of satisfaction, partaking of the nature of an incipient smile, and resembling that often seen on the face of man, could be plainly perceived in this animal. “The _Cebus azaræ_, when rejoiced at again seeing a beloved person, utters a peculiar twittering sound. It also expresses agreeable sensations by drawing back the corners of its mouth, without producing any sound. Rengger calls this movement laughter, but it would be more appropriately called a smile. The form of the mouth is different when either pain or terror is expressed, and shrill shrieks are uttered. Another species of Cebus in the Zoological Gardens when pleased makes a reiterated shrill note, and likewise draws back the corners of its mouth, apparently through the contraction of the same muscles as with us. So does the Barbary Ape (_Inuus ecaudatus_) to an extraordinary degree; and I observed in this Monkey that the skin of the lower eyelids then became much wrinkled. At the same time it rapidly moved its lower jaw or lips in a spasmodic manner, the teeth being exposed; but the noise produced was hardly more distinct than that which we sometimes call silent laughter. Two of the keepers affirmed that this slight sound was the animal’s laughter, and when I expressed some doubt on this head (being at the time quite inexperienced), they made it attack, or rather threaten, a hated Entellus Monkey living in the same compartment. Instantly the whole expression of the face of the Inuus changed; the mouth was opened much more widely, the canine teeth were more fully exposed, and a hoarse barking noise was uttered.