Atomaria linearis (or Atomaria nigrirostris?)

This proved a challenge to identify and I'm still not sure which it is, though provisionally I plumping for linearis. The spermatheca appears to make it nigrirostris but this is one of two species that should be separable from the other Atomaria (including linearis) fairly easily by the transverse keel at the base of the pronotum. I have stared at it from every conceivable angle under 63x magnification, dry and wet, but cannot see any trace of such a keel.

In the absence of a pronotal keel it didn't easily key and I kept hitting a dead end. The commonest Atomaria is meant to be linearis and photos of that species online looked a pretty good match, so after not getting anywhere working forwards through the key I tried to see if I could work backwards from linearis. The place where I'd diverged from the path to linearis was where I had to determine if the elytral hairs were shorter and hardly overlapping decumbent (linearis) or longer and clearly overlapping recumbent (not linearis). It seemed pretty clear to me that many of the hairs overlapped, which is why I went away from linearis. But they're also not lying flat on the surface so maybe that better fits the definition for decumbent than recumbent (the glossary definitions of the two terms overlap - "on or parallel to a surface" vs. "against a surface" - surely if a hair is against a surface it's also on the surface?). Anyway, there are some good quality photos of linearis on the internet and these show hairs that look very similar in length to my beetle - and overlapping just as much. So at this point I was reasonably happy mine was linearis.

Given the difficulties I thought it wise to check the genitalia matched linearis. Duff shows diagrams of the spermatheca for all the various Atomaria species and of all the diagrams the only one that was a really good match is the one for nigrirostris. But the next closest is linearis. The diagram for nigrirostris is the only one showing the coiled collum at the bottom as mine has. There's also a difference in the diagrams between linearis and nigrirostris in the shape of the middle section (the nodulus?). On linearis this section is shorter, and starts thick as it curves round from the top (cornu I presume), fairly abruptly narrowing towards the collum which starts before its level with the tip of the cornu. On nigrirostis, and on mine, the central section is longer, passing the tip of the cornu and starting to curve round before it turns into the collum (I'm taking the collum to start where it's evenly narrow). It tapers much more gradually. There's usually some subtle variation between individuals when it comes to spermatheca shape and I don't always expect an exact match in every respect, but here it is pretty much an exact match to nigrirostris and a couple of significant differences from linearis.

So in conclusion I think it's either a nigrirostris that's lacking a transverse keel on the pronotum or it's a linearis that's got a spermatheca that is a much better match for nigrirostris. Incidentally, the diagram of nigrirostris habitus (Fig. 58.9) in Duff shows a much more rounded front of the pronotum compared to my beetle, but photos of nigrirostris online show the same shape as mine (as do linearis).

The habitat described for nigrirostris in Hackston is completely different from that given in Duff - I don't know which is right but neither match where I found this. The habitat for linearis seems a bit more like where I found it - in a pitfall trap beside a pylon in the middle of an arable field (though no haystacks here and I don't think the current crop is beet).

female Atomaria linearis (or possibly Atomaria nigrorostris) showing spermatheca, elytral hairs, pronotum and antenna, Wendling Beck Environment Project (Norfolk, UK), 17th-18th May 2023