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The most important difference between the two groups of existing monkeys is indicated by their dentition. Adult man possesses, like all the other Catarrhine Simiæ, thirty-two teeth, whilst the American monkeys (the Platyrrhinæ) have thirty-six teeth--namely, one pair of premolars more in the upper and lower jaws. Comparative odontology leads us to the phylogenetic conclusion that this number has been produced by reduction from a still older form with forty-four teeth. This typical dental formula (three incisors, one canine, four premolars, and three molars, in each half-jaw) is common to all those most important older mammals which in the beginning of the Eocene period constituted the four large groups of Lemuravida, Condylarthra, Esthonychida, and Ictopsida. These are the four ancestral groups of the four main orders of Placentalia--namely, of the Primates, Ungulata, Rodentia, and Carnassia. They seem to be so closely related by their primitive organization that they may be united in one common super-order, Prochoriata.
With a considerable degree of probability, we are led to formulate the further hypothesis that all the orders of Placentalia--from the lowest Prochoriata upwards to man--have descended from some unknown common ancestor living in the Cretaceous period, and that this oldest placental form originated from some Jurassic group of marsupials.