M.C.A. 1-1-109 provides:
The common law of England, so far as it is not repugnant to or inconsistent with the constitution of the United States or the constitution or laws of this state, is the rule of decision in all the courts of this state.

Firman v. Firman, 610 P. 2d 178 - Mont: Supreme Court 1980 states:
At common law, a person could adopt any surname he might choose so long as the change was not made for fraudulent purposes...Title 27, chapter 31, parts 1 and 2, MCA, provides a procedure for changing one's name. These statutes are in affirmance and aid of common law and do not abrogate it. Kay v. Kay (Ohio Com.Pl. 1953), 112 N.E.2d 562.

Affidavit