I can not really find a definition, not even in a geologic dictionnary: What are articulate (resp. inarticulate) shells?
To be more specific, the term is related to tsunami deposits (tsunamite).
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If I remember correctly from my intro to paleo class, the terms articulate and inarticulate refer to a classification of brachiopods depending on the nature of their hinge. Articulate brachiopods have something like interlocking hinges (like a door hinge) made from serrated (or tooth like) parts of the shell. On the other hand, inarticulate brachiopods hold their shells together using only their muscles.
This means that the upper and lower shells of an inarticulate brachiopod will separate after it dies and its muscles decay. However, the shells on an articulate one will pretty much hold together unless a very strong mechanical force is applied. I don't know much about tsunami deposits, but this may have something to with that.
I'm not sure if this terminology extends to other phyla like molluscs, but I remember it from brachipods.
And here's a little picture because pictures make everything better.
Hope this helps!