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I have seven specimens of these, all with the same appearance and strange internal structure. They have perplexed me for years though I'm familiar with most fossils from the area. (I originally had three and have recently collected four more which enables them to be characterised more reliably. Altogether, five individuals are fairly complete). One small exposure of marine shale, Mississippian, Brigantian Stage, northern England, Co. Durham. The main features are: 1) See photos 3 & 4 -The shell material is strange, being irregularly honeycombed, resulting in a granular apperarance on the surface. I haven't seen any other shells at all like this from the same beds - brachiopods and bivalves being common. 2)They're roughly semicircular and shell-like. They very possibly have been paired down the straight edge. (Two of my seven specimens each consist of two disarticulated individuals, one on top of the other.) Bilaterally symmetrical, with a central sulcus or ridge. 3)The straight edge is thin and bent over. Where preserved it terminates with distinctive sharply curved ridged ornament. 4)They're all about the same size, 35-45mm across, and seem to be about 1mm thick. Although superficially brachiopod like, there's no sign of a hinge or umbo, and the texture isn't like any brachiopod I know. The fine structure closely resembles Jurassic ammonite aptychi of Laevaptychus type so aptychus is one possibility I've been considering though the shape doesn't match any I've found. Also, they're really quite common in the small source exposure whilst cephalopods are scarce (just one small orthocone to date). And Palaeozoic aptychi are generally rare and would be surprising. I have a few other ideas but will see what people can come up with. I hope it's obvious to someone! Here are four specimens, two of them being the possibly associated pairs. Thank you for looking! Specimen 1, paired: Specimen 1, top member removed (it broke off when drying!): Specimen two, worn fragment showing honeycombed internal structure. (All the specimens show this.) Specimen 2, closeup: Specimen 1, edge detail: Specimen 3, paired, partial at bottom overlaying complete one. Note close similarity of edge with previous photo (specimen 1) Specimen 4, with better preserved circular edge: EDIT: All seven specimens so far collected, in the same orientation to show their morphological consistency.