Mas"to*don (?), n. [Gr. masto`s the breast + 'odoy`s, 'odo`ntos, a tooth. So called from the conical projections upon its molar teeth.] (Paleon.) An extinct genus of mammals closely allied to the elephant, but having less complex molar teeth, and often a pair of lower, as well as upper, tusks, which are incisor teeth. The species were mostly larger than elephants, and their remains occur in nearly all parts of the world in deposits ranging from Miocene to late Quaternary time.

• Extinct elephant-like mammal that flourished worldwide from Miocene through Pleistocene times; differs from elephants and mammoths in the form of the molar teeth