with milk teeth and with permanent. The orbit is surrounded by a complete bony ring, posteriorly by a fronto-jugal arch, but still widely communicating with the temporal fossa. The placenta is diffuse and non-deciduous.
22. Stage of Simiæ. Orbit completely separated from the temporal fossa by an inward extension of the frontal and malar bones meeting the alisphenoid. Placenta consolidated into a disc, and with a maternal deciduous portion. Mammæ pectoral only. The dental formula is 2.1.3.3. All the fingers and toes are protected by flat nails. The tail is long. The American prehensile-tailed monkeys are a lower side-branch.
23. Stage of Catarrhinæ Cercopithecidæ. The dental formula is 2.1.2.3, owing to the loss of one pair of premolars in each jaw. The frontal and alisphenoid bones are in contact, separating the parietal from the malar bone; this feature is correlated with the enlarged brain. The internarial septum
