fenestella

1 plural -s a :  a niche like a window in the south wall of the sanctuary near the altar (as of a Roman Catholic church) containing the piscina and often also the credence b :  a small window or opening like a window (as in an altar front for allowing relics within to be seen)

2 capitalized [New Latin, from Latin, small window] :  a genus (the type of the family Fenestellidae) of Paleozoic bryozoans whose colonies form lacy fronds — compare lace bryozoan