The research described in this paper was begun by Mr. P. W. Palmer in 1912, at the suggestion of Prof. G. Elliot Smith. During the following year he published a short paper “On the Lower Jaw and Ear Ossicles of a Fœtal Perameles,” the specimen to which he referred being the 23·0 mm. greatest length fœtus mentioned below. Palmer also modelled the skulls of the 15·5 mm. and 16·0 mm. stages and the internal labyrinth of the 23·0 mm. fœtus, making notes on the work as far as he had taken it. On Mr. Palmer’s appointment to the Indian Geological Survey the work was handed over to me for completion. I am glad to have this opportunity of thanking Mr. Palmer for giving me the use of his models and notes; some of the latter are embodied in the present paper.