Abrocomidae Bathyergidae †Bathyergoididae Capromyidae Caviidae †Cephalomyidae Chinchillidae Ctenomyidae Cuniculidae Dasyproctidae †Diamantomyidae Dinomyidae Echimyidae †Eocardiidae Erethizontidae †Heptaxodontidae Heterocephalidae Hystricidae †Kenyamyidae Myocastoridae †Myophiomyidae †Neoepiblemidae Octodontidae Petromuridae †Phiomyidae Thryonomyidae Eucaviomorpha The Hystricognathi are an infraorder of rodents, distinguished from other rodents by the bone structure of their skulls.

The masseter medialis (a jaw muscle) passes partially through a hole below each eye socket (called the infraorbital foramen) and connects to the bone on the opposite side.

The 18 families within the Hystricognathi are divided into two parvorders, the Phiomorpha and the Caviomorpha.

Play behavior has been observed in seven hystricognath families.

They also rotate their heads and body muscles as a form of play.