There's a bit of a fine line between "extraordinary" and an "additional" or "extra ordinary" meeting. For ordinary business, i.e. business that would otherwise be handled in a routine meeting (and sometimes deadlines necessitate these matters be handled outside of the usual meeting schedule) then it is fine for the clerk to issue the summons but if the matter being considered is outside of the usual routine business, then the Chair or two councillors should issue the summons which must clearly state the business to be conducted at the meeting.