The dentition is diphyodont and heterodont, the incisors generally number 2/2, and the molars, except in the Hapalidae (Marmosets), are 3/3. The cheek teeth are adapted for grinding, and the molars are more complex than the premolars. A process from the jugal meets the postorbital process of the frontal completing the postorbital bar. The clavicle is well developed, and the radius and ulna are never united. The scaphoid and lunar of the carpus, and commonly also the centrale, remain distinct from one another. As a rule both manus and pes have five digits, but the pollex may be vestigial. The pollex is opposable to the other digits, and so is the hallux except in Man; the digits are almost always provided with flat nails. The humerus has no ent-epicondylar foramen and the femur has no third trochanter. The order Primates is divisible into two suborders: _Suborder_ (1). LEMUROIDEA. The skull has the orbit communicating freely with the temporal fossa beneath the postorbital bar (except in _Tarsius_). The lachrymal foramen is external to the margin of the orbit. Both pollex and hallux are well developed. In the pes the second digit is terminated by a long pointed claw, and so is also the third in _Tarsius_. The lumbar region of the vertebral column is long, sometimes including as many as nine vertebrae. Besides the Lemurs the group includes the aberrant _Tarsius_ and _Chiromys_. _Suborder_ (2). ANTHROPOIDEA. The skull has the orbit almost completely shut off from the temporal fossa, and the lachrymal foramen is situated within the orbit. The pollex is sometimes vestigial or absent. The second digit of the pes has a flattened nail except in the Hapalidae, in which all the digits of the pes except the hallux are clawed. The Anthropoidea are divided into five families: 1. Hapalidae or Marmosets. 2. Cebidae or American Monkeys. 3. Cercopithecidae or Old World Monkeys. 4. Simiidae or Anthropoid Apes. 5. Hominidae or Men. FOOTNOTES: [108] This is Gadow's view; according to Huxley the quadrate forms the malleus; according to Baur it forms the zygomatic process of the squamosal, and according to Broom the interarticular mandibular cartilage. [109] According to Leche, _Morphol. Jahrb._ XIX. p. 502, the molar teeth belong morphologically to the first series, i.e. they are milk teeth without vertical successors. [110] The researches of Bateson, _P.Z.S._ 1892, p. 102, have shown that cases of individual variation in the number of teeth are common. [111] Baur, however, suggests (_Anat. Anz._ vol. IV. 1889), that a tibial sesamoid found in _Procavia_, many rodents, edentates and _Ornithorhynchus_ is a vestigial tibiale, and that the astragalus is the intermedium. [112] This perforation of the acetabulum in _Echidna_ is a secondary character occurring late in development, and consequently is not of phylogenetic importance. [113] See R. Owen, "Monograph of the Fossil Mammalia of the Mesozoic Formation," _Pal. Soc. Mon._ 1871.