On 15 August 1959 Dr L. S. B. Leakey announced the discovery, in the lowest level of Olduvai Gorge, of a new and beautifully preserved fossil cranium of a hominid, which he tentatively named Zinjanthropus boisei. For this second volume, Professor Tobias has undertaken a definitive analysis of the cranium. This is a most comprehensive study to be made on a hominid skull belonging to the early and mid-Pleistocene group of australopithecines. The fossilised skull provides a wealth of information on taxonomic status, way of life and age at death. To evaluate Zinjanthropus fully, the author has reviewed in detail the cranial and dental anatomy of all australopithecines from Tanzania and South Africa and has placed on record much valuable information about the group as a whole.