"The essential ingredients of a shell structure are continuity and '. ...an ancient masonry or vault is not obviously continuous... it may be composed of separate... sub-units or s not necessarily cemented... But in general... are held in a state of compression throughout... thus in compressive contact... [S]hells are structurally continuous in the sense that they can transmit forces in a number of different directions in the surface of the shell, as required. These structures have a quite different mode of action from skeletal structures... [which are] only capable of transmitting forces along their discrete structural members."