618 THE PRIMATES xxm. 7 



the type has been successful and is expanding. The outstanding 

 characteristic of the anthropoids might be said to be their liveliness 

 and exploratory activity, coming perhaps originally from life in the 

 tree-tops, necessitating continual use of eye, brain, and limbs. With 

 this is associated the development of an elaborate social life, based not, 

 as in most mammals, on smell, but on sight. Monkeys show more 

 bright colours than do other mammals, especially the curious reds and 

 blues worn on the head and rear. The species may become subdivided 

 into distinct races showing great differences of coat colour (Fig. 400). 

 Communication between individuals is ensured by elaborate systems 

 of vocal signals and the platysma muscle becomes differentiated into 

 a set of facial muscles used to signal 'emotions'. 



Many of the characters of the group are those of Tarsius, listed 

 already, but the Anthropoidea are mostly diurnal and microsmatic, 

 with a short snout, large forwardly-directed eyes, many cones, a well- 

 marked central area in the retina and partial decussation in the optic 

 tract, features that are associated with binocular vision and large powers 

 of visual form discrimination. The external ears, no longer serving as 

 tactile organs or for direction-finding, are small and the edge is usually 

 rolled over. The orbit is closed off behind. The tympanic bone is 

 fused to the petrosal and in later forms drawn out to a spout. The 

 tactile sense is greatly developed on the fingers and toes, which carry 

 characteristic ridges. The brain (Fig. 386) is relatively much larger 

 than in lemurs or Tarsius and its cerebral hemispheres are especially 

 well developed, overhanging the cerebellum and medulla. The olfac- 

 tory parts of the brain are reduced and the pyriform lobe becomes 

 displaced on to the medial surface by the extension of the neopallium. 

 The surface of the neopallium is highly fissured, showing a charac- 

 teristic form of Sylvian fissure, and a well-marked central sulcus, 

 separating the motor and sensory areas. A parieto-occipital sulcus 

 separates these lobes and a large lunate sulcus marks the visual area, 

 especially in monkeys (simian fissure). The neocortex thus shows four 

 distinct lobes, frontal, parietal, occipital, and temporal. The occipital 

 (visual) and frontal regions are especially large. 



The head is rounded to fit the brain, with the foramen magnum 

 below, so that the head is carried up on a mobile neck. The gait of 

 monkeys is typically quadrupedal and plantigrade when on the ground, 

 with the fore-limbs somewhat longer than the hind. In the trees 

 some may be described as low canopy runners (e.g. guenons) others 

 are high canopy acrobatic types (spider monkeys). Only the great 

 apes habitually swing along with the arms (brachiation). Some 



