Abstract
A new species of Megascyliorhinus (M. trelewensis) comes from the Monte León Formation (“Patagoniano” partim; upper Oligocene–lower Miocene) of eastern Patagonia. It is compared with other species of Megascyliorhinus, in particular M. miocenicus, although there is no implication of relationship to the latter. An Eocene–Pleistocene size increase in the species of Megascyliorhinus appears to be demonstrated. The supposed relationships with Megachasma pelagios are discussed.