LSJ registers perikti/ones as an epic/poetic nominative plural (very rare in prose) and declares that the singular 'is not in use'. Nevertheless, the present entry as transmitted appears to be an instance (extracted from somewhere) of its genitive singular; and, differently accented (periktio/nos), grammarians note it also.
For the glossing cf. e.g. the scholia to periktio/nwn in Thucydides 3.104.3.