簡単な平面図形の名称

Plane figures with some segments and arcs or circles are very easy to draw down, yet thereare some of them which have no specified names. I picked up one of them here. It is acombination of a sector and a circular ring; see the figures 3, 1 and 2. This combinationshown in the figure 3 is now called to be a part of a circular ring, but this does not showits idea distinctly. I could not find the proper name for this figure neither in English norin Japanese. I name it a sector base or a sector subring. I could also name it a circularsubring, but in doing so it is very confusing with the word subring used in abstract algebra.