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The specialized masticatory apparatus of Permo-Triassic dicynodonts (herbivorous mammal-like reptiles) is described and its function analysed by comparison of an archaic dicynodont, the pristerodont Emydops, with the advanced dicynodont Lystrosaurus. A unique feature of the dicynodont masticatory apparatus was the fact that the jaw joint consisted of two convex articulating surfaces which permitted free anteroposterior movement. Food was broken up during retraction—protraction serving merely to move the jaw forward for another retractive stroke. True propalinal action did not occur. Food was comminuted by shearing; grinding or crushing was not an integral part of the masticatory cycle. In Emydops, cutting took place at the beak when the jaw was elevated by the vertical component of force of the external adductor muscles. Slightly later in the masticatory cycle, cutting took place at the dentary teeth as the jaw was retracted by the stronger horizontal component of force from the same muscles. The two cutting areas and the motions by which they were utilized are distinct. In Lystrosaurus dentary teeth are lacking and the cutting area at the front of the jaws is expanded. Motion of the lower beak consisted of a combination of elevation and retraction by the powerful horizontal component of the muscles. During much of the Upper Permian and Lower Triassic, when dicynodonts were the dominant tetrapod herbivores, their peculiar masticatory apparatus enabled them to invade an environment apparently barred to conventional herbivores. The line leading to Lystrosaurus probably originated from an Emydops-like form, the main trend of evolution being toward increasingly effective utilization of the horizontal component of force exerted by the external adductors. However, the pristerodonts persisted with little change during most of the interval in which this evolution took place. It appears that the archaic jaw mechanism of pristerodonts was adapted in some way to a persistent environment which was different from the environment that favored the Lystrosaurus type of specialization.