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 $ and may be 4; 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, z.e., the Lemur- 
oidea and Anthropoidea. 
SUB-ORDER I.—LEMUROIDEA. 
The Lemuroidea unquestionably rank lower than the 
other sub-order. They are more quadrupedal and in 
Eocene strata they appear to gradate into the Jusectivora. 
They differ from the Avthropoidea 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. 
