“…Gangloff and Gustafson (2000) proposed that large portions of Montana's warm-water river reaches may serve as nursery refuges for non-native mussel species that are imperiled elsewhere. More recently, a newly discovered black sandshell population in Canada has extended its range further north and west in the province of Saskatchewan (latitude 53°4 N, longitude 106°00W) (Phillips et al 2009), but the authors do not speculate if this is an introduced population or undetected in prior surveys. Here, we report collections of the black sandshell from three locations on the Missouri (latitude 47°7 N, longitude 110°8 W) and Marias Rivers (latitude 47°9 N, longitude 110°5 W), that are approximately 26 river kilometers west (upstream) of any previously documented Missouri River occurrences and newly reported for the Marias River watershed (Table 1, Fig.…”