The leaves are very small compared to all the species in the BBS Field Guide, so this plant may be stunted or immature. The shoot size was only 3 or so centimetres. and I only had a handful of leaves to work with. I do have a reserve plant though which I can re-hydrate to check leaf decurrence again. Nerve is clearly excurrent.
Some images first
|Is that decurrent? Maybe a bit? Not hugely though|
|only c.2.5mm leaf length, by c.1.5|
|Toothed upper leaf|
|Can we call that diagonal? I'd say so|
|One full unit is 25 microns. No obvious Diagonal here, oddly|
* it's not Rhizomnium punctatum, because it has a toothed upper leaf. One down!
* it's not P.cuspidatum because it's not that toothed.
* P.affine, P.elatum, P.medium all have decurrent leaves
* P.affine is quite strongly toothed, has cells in diagonal rows. I'd rule it out on teeth
* P.ellipticum has diagonals; does not have decurrent leaves
Back to P.rostratum, the Field guide differentiates other spp on diagonal cells, but many leaf images of rostratum online do show diagonal cell arrangement, so I think that's a red herring. Maybe it's a handlens vs. microscope thing. The FG says Affine differs in having cells "longer than wide". Well, in some parts of the leaf this is true and in others not so much!
So, no conclusion yet. Final decision might be in abeyance until somebody can give me a steer.
BBS Field Guide P.rostratum
BBS Field Guide P.affine
BBS Field guide P.ellipticum
BBS Field Guide P.cuspidatum
Nice link to P.ellipticum images