But on examining the hands carefully, and noticing the deep parts as well as the outside, it was found that they could be ranged into two sub-groups. In one there is no external appearance of a thumb, and in the other there is a stunted projection, but in both the member is not quite deficient so far as its bones are concerned. In the first group the metacarpal bone (the bone which is in man covered by the ball of the thumb, and which extends from the wrist to the first joint) is just seen, but it does not project; and in the second group there is one phalanx or thumb-bone on the metacarpal, and this sticks out and is covered with skin so as to resemble a hard pimple. In one kind this little thumb has no nail, and in another there is one on it. It is curious that some of the woolly-haired kinds of Ateles should have no thumbs and others their rudiments; and that this should be the case in the long and harsh-haired kinds also. There are many kinds of Ateles, and there is consequently some difficulty in recognising them as species and many attempts have been made to classify them, so that they might be readily distinguished. Those with short and thick thumbs have been called Brachyteles, and those without them Ateles; those with woolly fur have been termed Eriodes, but all are now included in the genus Ateles. Everybody is interested in seeing the curious sprawling swinging of the Ateles in the Zoological Gardens, and also in noticing the curious way in which some can place their hand right over the head nearly on to the opposite shoulder, and brush the hair with it forwards, and especially because both kinds of movement refer to the great length of the fore-limbs. On the contrary, although they can maintain the erect posture for a short time, they seem feeble about the hind limbs, which are shorter than the others. Their heel-bones are evidently short, so that leaping is never well done. [Illustration: BRAIN OF THE SPIDER MONKEY.] They are fruit and vegetable eaters, and enjoy eggs, and a nut occasionally, but they have no cheek-pouches. They have, however, an air-pouch, or sac, in front of the throat, but none of the noise-making gifts of the Howlers, and this sac enters the windpipe differently to those of the Monkeys of the Old World, and this is very curious. It opens into the windpipe below the cartilage which forms the “Adam’s apple” in man, and not above and between it and the tongue. Below this cartilage, which is called the thyroid cartilage, there is another attached to it by which it joins on to the rings of the windpipe. The opening is between this lower cartilage, the cricoid,[83] and the top ring of the windpipe.