Assertion: oyster's shell | /r/HasProperty | upper one flat
Facets:
Source sentences:
text: The shell is made up of two valves, the upper one flat and the lower convex, with variable outlines and a rough outer surface.
text: Oyster shell are irregular in shape and the shell is made up of two valves, the upper one flat and the lower convex, with variable outlines and a rough outer surface.