If you find a relatively robust, usually upright Plagiomnium with ovate leaves growing in a wet, lowland base-rich marsh, spring, flush or fen, it is most likely to be P. elatum or the scarcer P. ellipticum. Check with a hand-lens for obviously decurrent leaf bases – if they are broad (sometimes very) and quite obvious, you will have P. elatum. P. ellipticum has scarcely decurrent leaves and the same habit as P. elatum. P. affine, a species with longly but very narrowly decurrent leaves usually creeps around in drier places. All three species have porose mid-leaf cell walls.
Sometimes it can also be helpful to look at the structure of the marginal teeth, a character that not enough is made of in the floras. P. elatum has sharp teeth 1-3 cells long that stick out at a wide angle from the leaf. The teeth of P. ellipticum are usually only 1 cell long.Read the Field Guide account