Hystricognathi
The Hystricognathi are an infraorder of rodents, distinguished from other rodents by the bone structure of their skulls. The masseter medialis passes partially through a hole below each eye socket and connects to the bone on the opposite side. This, together with their lack of an infraorbital plate and the relative size of the infraorbital foramen, distinguishes hystricognaths from other rodent groups. The infraorder's name comes from Ancient Greek ὕστριξ, meaning "porcupine", and γνάθος, meaning "jaw".
The 18 families within the Hystricognathi are divided into two parvorders, the Phiomorpha and the Caviomorpha. The Caviomorpha are mostly native to List of [mammals of South America#Order: Rodentia (rodents)|South America], with a few species in the Caribbean and List of [North American mammals#Rodents|North America], while the Phiomorpha occur in the Old World.
Image:Nutriaschädel.jpg|thumb|left|250px|Skull of a nutria demonstrating the hystricognathous lower jaw and hystricomorphous zygomasseteric system