Anguloserra sp.
Brigantian (Mississippian) marine shale, Co. Durham, UK.
I was scanning a lovely hash plate of bryozoan and Archaeocidaris bits under a binocular microscope when this odd but very distinctive piece leapt out at me.
They don't seem to have been recorded from the UK Carboniferous but I had found one a few years ago at the same locality and it took ages to track down an ID.
It's two overlapping goniodonts or teeth from an ophiocistioid.
This is a group of strange Palaeozoic echinoderms known largely from their tooth elements which form overlapping ranks, a good overview is here:
http://echinoblog.blogspot.co.uk/2012/08/ophiocistioids-weird-mystery-fossils.html (One like this is pictured about halfway down).
Hash plate jumble, the goniodont pair is arrowed.
Brass scale is 1cm.
Overlapping pair of goniodonts pointing downwards, next to a couple of Archaeocidaris spines.
Closeup: