Paired with https://www.inaturalist.org/observations/18216629. A little more detail.
M.truncata is closely related to M.arenaria - picture1. The similarities are not entirely obvious but the differences are strong.
The truncated posterior seems ill-designed, an invitation for predator attack.
I love investigating angles and contours - there is much to photograph in low level daylight.
Added 15 Nov
Picture three is all my own work, in as much as this perspective on angle is not displayed or discussed anywhere I could find - it may or may not be useful for ID but the angle displayed varies very little in all the specimens I have found.
All but ONE, in fact, because I have located a shell with the truncation in the opposite hand. I will post it soon for interest.