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Hystricomorpha (cohort Glires, order Rodentia) A suborder comprising many families of rodents, in which the medial masseter muscles spread forward through the infra-orbital foramen, while the lateral masseter is attached to the zygomatic arch, as in primitive rodents. The suborder includes the porcupines of the Old and New Worlds, guinea-pigs, capybara, agoutis, vizcacha, mole rat, etc.