Curtonotus aulicus

I don't recall having any difficulty identifying this species before but this one didn't key quite so smoothly. Unless I'm misinterpreting things somehow, the prosternal process appears to have a thin margin. As such it keys to Amara apricaria, but at twice the size. It clearly isn't Amara apricaria (one of which I caught the same night and examined at the same time), and looks just like Curtonotus aulicus, so I'm not really in any doubt about the ID but what's the ridge-like thing round the perimeter of the prosternal process if it's not a margin?

Curtonotus aulicus showing close-ups of prosternal process, North Elmham (Norfolk, UK), 23rd July 2020

After examining that one, I made sure I had a good look at the prosternal process on the next one I found - and this one did indeed have the margin missing at the apex, just as it's supposed to.

Curtonotus aulicus showing close-up of prosternal process and pronotal hind margin, Wendling Beck Environment Project (Norfolk, UK), 12th June 2023

The next one may have helped clear up the mystery with the prosternal process seemingly being margined round the apex. On this one the margin did not continue round the apex, but there was a slight step where the surface of the apex started to curve round at the apex, and at some angles this made it looked margined. Maybe that's what was going on with the first one above?

Curtonotus aulicus showing close-up of prosternal process, North Elmham (Norfolk, UK), 10th August2023

Curtonotus aulicus showing close-up of prosternal process and pronotal hind margin, Wendling Beck Environment Project (Norfolk, UK), 11th August2023

Curtonotus aulicus, North Elmham Cathedral Meadows (Norfolk, UK), 4th September 2018

Curtonotus aulicus, North Elmham (Norfolk, UK), 12th April 2018

Curtonotus aulicus, North Elmham Cathedral Meadows (Norfolk, UK), 16th April 2019