I was always told these were brachiopods
Highlands Intermediate CoastBlitz Tapuae
Often you can see vein like patterns through the outside of these shells. They are quite robust in the sense that they easily seem to stay whole.
At Elkhorn Slough National Estuarine Research Reserve
Common Red Lamp Shell.
Second shot depicts both valves inside and out.
How about this striated species of Lamp shell, and no baby at 35mm, in fact huge [highlighted specimen], fossicked from Manawa-kapakapa [spelling ?] south end of D'Urville Is.
Does not appear to be same sp as the smoothe shelled Common Red Lamp Shell [Magasella sanguinea] but may be a variation on it. Both this, and the Common Red Lamp Shell were found same beach or Ohana Bay over peninsula in Current Basin. [Next posting]
Brachiopods, phylum Brachiopoda, are marine animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized, articulate and inarticulate. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system...