Mont. Code § 2-1-207
Exclusive jurisdiction shall be and the same is hereby ceded to the United States over all that part of territory situate in the state of Montana now embraced in the Yellowstone national park, described as follows:
Beginning at the intersection of the east boundary of Yellowstone park with the south boundary of Montana; thence north to the northeast corner of said park; thence west along the north boundary of the park to the northwest corner thereof; thence south along the west boundary of the park to the boundary between Montana and Idaho; thence easterly along that boundary to the west boundary of Wyoming; thence north along the west boundary of Wyoming to the northwest corner thereof; thence east along the boundary between Wyoming and Montana to the east boundary of said park, the place of beginning; containing an area of approximately 198 square miles, saving, however, to the said state the right to serve civil or criminal process within the limits of the aforesaid described lands in suits or prosecutions for or on account of rights acquired, obligations incurred, or crimes committed in said state but outside the lands aforesaid, as long as the lands herein described are used for a national park and no other purposes.
§ 2-1-207, MCA