As these Guenons walk on all-fours and but rarely take on the erect posture, which, moreover, they cannot maintain, their muscles are not exactly the same as in the Troglodytes and Orangs, but they resemble those of the Semnopitheci. The Guenons, like the Macaques and Baboons--those great runners on all-fours--have a special muscle to assist in pulling the shoulder-blade forward, and thus to assist the forward motion of the body. Then, in order to drag the elbow backwards in moving on all-fours, and to assist also in climbing, one of the large muscles of the back sends a slip to the back of the elbow. Climbing is also assisted by an addition to the gluteal or buttock muscles, which is called the scansorius or climbing muscle. And in the foot the front muscle of the leg has two masses; one sends a tendon which goes to the inner and front bone of the ankle, and the other right under the foot to the inner side of the long bone (metatarsal), which supports the toe-thumb. The result of its action is to turn in the foot with a view to holding on. Finally, the two long muscles which flex or bend down the toe-thumb and the other toes are not separate, but are connected by their tendons. So that there is not great independence of the toe-thumb, but all the toes act more or less simultaneously very readily. But the other muscles of it give it more mobility than in man. Their muscular energy is immense, and their power of using the thumb is very considerable, and they pick out each other’s vermin with well-known ease. In separating the numerous kinds of Guenons into kinds or species, paying a good amount of attention to their internal as well as external structures, that is to say, to their teeth and skull, as well as to their form, it becomes evident that some large ones form a group which closely resemble the others, but which still have more general likeness to the Monkeys which form the subject of the next chapter--the Macaques. These have been placed in a separate genus, but the necessity for doing so is not apparent, especially when the principles of the true nature of classification have been thoroughly comprehended. So the so-called genus Cercocebus (κέρκος, tail; κῆβας, monkey) is omitted, and the Monkeys included in it by some authors are to be considered as the kinds which link on the Baboons and Macaques to the Guenons. Besides these, some Guenons are stronger and stouter than others, their skins being green, or tinted more or less with that colour, and another is of a bright red colour. So that several sets of the Guenons may be established for the sake of convenience--1. The smaller kinds usually with prominent white markings. 2. One having a green skin and a black nose, and only three points or cusps on its hind lower molars. 3. The larger kinds with decidedly green tints, one being bright red. 4. And the group often called Cercocebus, which resemble the others, but have a fifth cusp on the last lower grinder on each side.