The Montana Arctic grayling (Thymallus arcticus montanus) is a North American freshwater fish in the salmon family Salmonidae. The Montana Arctic grayling, native to the upper Missouri River basin in Montana and Wyoming, is a disjunct population or subspecies of the more widespread Arctic grayling (Thymallus arcticus). It occurs in fluvial (or river–dwelling) and adfluvial, lacustrine (or lake–dwelling) forms. The Montana grayling is a species of special concern in Montana and had candidate status for listing under the national Endangered Species Act. It underwent a comprehensive status review by the U.S. Fish and Wildlife Service, which in 2014 decided not to list it as threatened or endangered. Current surviving native populations in the Big Hole River and Red Rock River drainages represent approximately four percent of the subspecies' historical range. Taxonomy The scientific name of the Montana Arctic grayling, when treated as a subspecies, is Thymallus arcticus montanus. It was first described in 1874 as Thymallus montanus by fisheries biologist James W. Milner. The type specimen was collected by Acting Assistant Surgeon, George Scott Oximixon, U.S. Army near Camp Baker, Montana Territory. Camp Baker was located very close to the Smith river northwest of White Sulphur Springs in Meagher County. Milner also based his description on three specimens taken by geologist Ferdinand V. Hayden during his Geological Survey of 1872 from Willow Creek in Gallatin County, Montana. North American conservation and fisheries authorities generally do not cite the Montana grayling as a subspecies but a distinct population segment of the Arctic grayling Thymallus arcticus. Description The Montana Arctic grayling has a large, sail-like dorsal (back) fin and colorful body markings. The grayling comes in a wide array of colors. Their dorsal fins are typically fringed in red and dotted with large iridescent red, aqua, or purple spots and markings. These colorful markings are most dramatic on large grayling. Graylings’ backs are usually dark. Their sides can be black, silver, gold, or blue. A band of gold forms a border between their sides and white bellies, which are in sharp contrast to their pelvic fins striated with iridescent orange, red, or pink. The sides of the body and head can be freckled with black spots. A black slash lies on each side of the lower jaw. The iris of their eyes is often gold in color. Only their adipose, caudal (tail), pectoral, and anal fins are without much color; typically dull