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Sciuromorpha (Protrogomorpha; cohort Glires, order Rodentia) A suborder comprising the more primitive rodents, in which the masseter muscle is attached mainly at the lower edge of the zygomatic arch; and more advanced forms, placed in the family Sciuridae, in which the middle masseter is attached at the outside of the skull in front of the orbit. The suborder includes 13 families of beavers, squirrels, marmots, and gophers. Some authorities prefer the term ‘Protrogomorpha’, which indicates the primitive level of development of members of the suborder, the sciurids being specialized forms rather than central to the group.