MAMMALIA. 587 



generalised and belong to the transition arboreal group. 

 The possibilities of movement in the pentadactyle limb and 

 vertebrate skeleton are seen in this order at their maximum. 

 Many of the order are omnivorous, though a frugivorous or 

 insectivorous diet is common. The incisors are usually 

 reduced to f and may be \ ; they are commonly chisel-shaped. 

 The canines are mostly longer than the incisors and nearly 

 always present. The cheek-teeth are usually quadrituber- 

 culate and have flat grinding crowns. 



In the limbs the five digits are usually all present and 

 the hallux is with one exception opposable to the other 

 toes (arboreal). The claws have a tendency to become 

 flattened into nails. The radius, ulna, tibia and fibula are 

 all complete and the full movement of supination and pro- 

 nation is retained. For similar reasons the clavicle is always 

 well developed and there is little or no fusion of the tarsal or 

 carpal bones. Terrestrial locomotion is plantigrade. 



The orbits tend to face forwards instead of laterally 

 and they are always complete. 



The brain is highly developed, the cerebrum being much 

 convoluted and covering the cerebellum. Its proportion to 

 the body is very high (see page 463). 



The placenta is either diffuse and non-deciduate or 

 metadiscoidal and deciduate. 



The Primates are, like a good many other preceding 

 orders, sharply divided into two sub-orders, i.e., the Lemur- 

 oidea and Anthropoidea. 



SUB-ORDER I. LEMUROIDEA. 



The Lenmroidea unquestionably rank lower than the 

 other sub-order. They are more quadrupedal and in 

 Eocene strata they appear to gradate into the Insecfivora. 

 They differ from the Anthropoidea in the invariable 

 presence of all five digits, in the lengthened facial region 

 of the skull, the orbit being only separated from the tem- 

 poral fossa by a (postorbital) bar of bone, not a partition, 

 and the lacrymal foramen being outside the orbit, in the 

 lower type of brain with smaller and less-convoluted cere- 

 brum, in the possession of a diffuse, or dome-shaped, non- 

 deciduate placenta and somewhat bicornuate uterus. 



