I found this Archaeocidaris (probably A. urii) plate last week and have already shown the little Fenestella on it.
It's from the Brigantian of County Durham, UK.
As well as plates and spines, it has some scattered Aristotle's lantern elements including teeth.
Whole slab, with two complete teeth and one broken, arrowed in red (there are others).
Other lantern elements and sundry small plates in the red oval.
Tooth (the one on the right in photo 1). This seems large at about 4.5 mm across.
Closeup of tooth tip:
Area in oval on photo 1, showing other lantern elements with small peristomal plates scattered around. The large ear shaped piece at bottom right is an epiphysis from the lantern.