This paper describes a new mammalian tooth from the Lower Wealden bone-bed at Cliff End, near Hastings. The tooth is a lower molar. It has a trituberculate structure with a well-developed talonid. The nature of the wear on this talonid shows that a true protocone (a definite pointed cusp as against an enlarged lingual cingulum) must have been present in the upper molars. Aegialodon is the first mammal in which the existence of a definitive protocone can be inferred. A good morphological series can be traced back from Aegialodon through Peramus (Upper Jurassic), Amphitherium (Middle Jurassic) to the Upper Triassic Welsh pantotheres. This series illustrates the increasing importance of the crushing as against the shearing function of the molar teeth as their evolution progresses. The morphological expression of this development is the enlargement of the talonid and associated enlargement of the lingual cingulum of the upper molars. Finally, in the last member of the series (Aegialodon) the lingual cingulum becomes a true protocone. From this main line of therian evolution first the symmetrodonts and then the dryolestid pantotheres branched off by emphasizing the shearing function of the teeth and reducing their crushing function. This would have been an adaptation to an exclusively insectivorous diet. From Aegialodon can be derived the teeth of the Trinity Sands 'Theria' and Endotherium by a further increase in the size of talonid and protocone. A continuation of the same process would give rise to the teeth of marsupials and placentals.