"A shell can be defined as a body that is bounded by two surfaces parallel to its middle surface, and is deformed in any arbitrary manner. This is true for shells of a constant thickness... considered in this study. ...One particular way of classifying shell surfaces is according to their Gaussian curvature. \kappa_g = \kappa_1 \cdot \kappa_2 = \frac{1}{r_1} \cdot \frac{1}{r_2}Another way of describing shell surfaces is according to how the surfaces are generated. Using this method, in 1980 classified shell surfaces into Geometric, Structural and Sculptural surfaces. Geometric shells are well defined mathematically and... easily calculated analytically. These type of shells were quite significant in the development of shell structures [when] computer[s]... were not available. ...geometrically shell surfaces can be classified as cylindrical... spherical... conical... paraboloidal... etc."