Final answer:
The principal member of a cornice that crowns the wall is called a corona. It is part of the entablature in classical architecture, which includes the cornice, frieze, and architrave.
Step-by-step explanation:
The principal member of a cornice which projects like a shelf and crowns the wall is called a corona. This term refers to the projecting ledge that caps the top edge of a wall, often found in classical architecture. The entablature is the structure above the columns that supports the roof, which is comprised of three main parts: the cornice, the frieze, and the architrave.
The corona is specifically the outward-facing part of the cornice that overhangs the rest of the entablature, providing both functional protection from the elements and aesthetic value to the structure.