Silpha tristis

The only place I really struggled keying this was ruling out atrata - at some angles the elytra looked matt but at others it looked a bit shiny, especially viewed under the micrscope. And the head shape I wasn't completely convinced about either without anything to compare it to. I was pretty sure it wasn't atrata though - I'd seen atrata before and don't recall struggling over them, and the habitat suited tristis much better. Pretty sure, but I needed to be completely sure. Mike Hackston's key mentioned a difference in mandible shape which isn't in Duff (and helpfully illustrates it for atrata) and with that I was pretty happy. The final clincher came when I looked at Adrian G's beetle collection in which he had both species. With the two species side-by-side the differences were actually really obvious!

Silpha tristis showing head (close-ups of labrum and antenna) and elytra (including close-up of punctures), between Titchwell and Thornham Point (Norfolk, UK), 10th June 2022