Mitrula borealis seems to be very common in SW Finland, perhaps more so than M. paludosa. Some of my observations are recorded here, but I have many more:

https://laji.fi/observation/list?target=mitrula%20borealis

A few weeks ago I collected a Mitrula with spores even broader than borealis according to the Redhead paper. I don't know is it a borealis or still a new one.

About macroscopic differences I am not sure but the working hypothesis is that borealis is more yellow and perhaps bigger. Paludosa might have more brownish tints in the hymenium and pehaps not so elongated hymenial head as borealis.

Likewise, about ecology I have only a working hypothesis: borealis might be more common in spring fed small streams or pools while paludosa is found in any pool or ditch. All acid, virtually no calacreous sites where I collect.

The sequenced one is this:

http://tun.fi/JX.1258947#9

Yes, only overmature spores are septate. The spore sizes in the prevoius image are all the same as in the observations in the first link in this thread, if you scroll a bit to the right or look at the observation details.

Hello,
interesting topic! I must admit I have automatically identified all my Czech collections as M. paludosa. In Czech literature, I haven´t found a mention about M. borealis, but I can see that Redhead mentions occurrence in the neighbouring Germany.
I wonder how often is M. borealis collected in this region... 
Yesterday I found Mitrula in the Jeseníky Mts., so I made the first microscopy of it in my life, and it fits better M. paludosa, especially if Redhead worked with dead material. In any case, the are thinner than those in the collections from Finland presented here. Living ascospores measure (13)14.1-18.2(19.2) × (2.8)3-3.6 (3.8) µm, Q = (3.8)4.2-5.6(6.2), n = 40. Me = 15.8 × 3.3 µm; Qe = 4.8. Gelatinous sheath not observed.
Best regards, Zuzana

Thanks for all the replies. Interesting. I don't have the papers now, but I might access them. I have a sample of this drying right now, and perhaps a DNA sequence will be made of it.

My Mitrula grew in a little brook, on detritus of Picea and Acer pseudoplatanus, most often directly on Picea cones. Altitude is 845 m a.s.l.

For the sake of comparison, here is what is presumably Mitrula paludosa, found a couple days ago from another location. The spore shape is a bit narrower, and the fertile part of the fruitbodies was on average shorter than with the former finding (which is presumably M. borealis).

Spores:

(17.3) 18.1 - 21.3 (22.9) × (2.8) 3 - 3.5 (3.8) µm
Q = (4.6) 5.4 - 6.7 (6.8) ; N = 20
Me = 19.6 × 3.3 µm ; Qe = 6 

My guess would be that both are paluosa despite the spore length difference, but I do not know for sure.