23mm. The pattern is more jumbled.
The shell was found on top of the outer flank of the path, in its tuft of weeds.
In this portion, shells are numerous, and sorted by species:
a batch of Callios, a batch of Ocenebras, Callios again, etc.

This outer flank is a sloping wall, on which rest many Callios and Ocenebras.
The opposite side of the path delimits a tide pool colonized by Gibbulas & Ocenebras,
while some rare Callios crawl in fissures under our feet: the causeway, here, begins to decay.

Ocenebra erinaceus (Linnæus, 1771). 26-27mm.
The flat surface of the path is like a kind of solarium on which the shells sleep.
Sting Winkles are usually bigger, but these ones are not very aged; old timers remain in the water.

Kind of frills. Most of these shells are encrusted, especially around Saint-Malo.
A fully adult specimen, with apex and no barnacles or worms, is something I never met here.