I say all this because I have a plant in my field collection box marked as D.tophaceous, but it has rounded toothed tips. Every day's a school day, as they say.
Some borrowed images from Oregon State Uni:
My "Didymodon". The leaf below is really about 0.9mm - a very small example, so potentially confusing. Cell shape looks pretty Racomitrium to these newbie eyes too.
And a not-so-great shot of the plant as a whole
(Just noticed on Idaho website, comment on the Didymodon - <<I first found this species in Idaho in 1975, at which time I noted on the packet, "like Racomitrium aciculare except the margins are bistratose">> - so the similarity is noted there too)