Mont. Code Ann. § 5-2-212

At 12 noon on the day appointed for the meeting of any regular session of the legislature, the senior member present must take the chair, call the senators and senators-elect to order, call over the senators from the certified roster prepared by the secretary of state, and then, from the certified roster prepared by the secretary of state, call over the senatorial districts and counties, in their order, from which members have been elected at the preceding election. After the same are called the members-elect must take the constitutional oath of office and assume their seats. The senate may thereupon, if a quorum is present, proceed to elect its officers.