Cercyon ustulatus

This species is separated from most of the others in the genus by not having a smoooth curve in side view, there being a slight dip between the curves of the pronotum ad of the elytra. However this was so slight I wasn't convinced it was enough. Supporting characters include the side elytral intervals which were slightly convex towards the apex (as detailed by Duff) and weakly convex on the sloping sides (as detailed by Hackston). As a double-check I tried keying it as if it wasn't ustulatus but eventually hit a dead end (with neither of the last pair of species remaining looking anything like this). Although there are other Cercyon species that have black elytra with a pair of reddish spots at the rear, most don't look quite like mine although bifenestratus is pretty similar (bifenestratus is easily eliminated as it has a broader, more oval, mesosternal field; the similar marinus has a more similar mesosternal field to ustulatus but has the orange on the elytra extending up the edge to close to the humerus).

Cercyon ustulatus showing side of elytra and mesosternum (2 views), Wendling Beck Environment Project (Norfolk, UK), 8th September 2023