arcade had been proposed by Bramante for the Palazzo di Giustizia (unexecuted), and was employed by Antonio da Sangallo the younger for the courtyard of Palazzo Farnese (begun 1517). In adopting the solution for the Marciana Library, Sansovino was faithfully adhering to the recommendation of Leon Battista Alberti that in larger structures the column, inherited from Greek architecture, should only support an entablature, whereas the arch, inherited from Roman mural construction, should be supported on square pillars so that the resulting arcade appears to be the residual of "a wall open and discontinued in several places". According to the architect's son, Francesco, Sansovino's design for the corner of the Doric frieze was much discussed and admired for its faithful adherence to the principles of Ancient Roman architecture as outlined by Vitruvius in De architectura. These principles required that a triglyph be centred over the last column and then followed by half a metope, but the space was insufficient. With no surviving classical examples to guide them, Bramante, Antonio da Sangallo the Younger, Raphael, and other great Renaissance architects had struggled with the dilemma, implementing various ideas, none of which satisfied the Vitruvian dictum. Sansovino's solution was to lengthen the end of the frieze by placing a final pilaster on a wider pier, thus creating the space necessary for a perfect half metope. Francesco Sansovino relates that his father additionally sensationalized the design by challenging the leading architects in Italy to resolve the problem and then triumphantly revealing his own solution. Carvings Rather than a two-dimensional wall, the façade is conceived as an assemblage of three-dimensional structural elements, including piers, arcades, columns, and entablatures layered atop one another to create a sense of depth, which is increased by the extensive surface carvings. These are the work of Sansovino's collaborators, including Danese Cattaneo, Pietro da Salò, Bartolomeo Ammannati, and Alessandro Vittoria. Male figures in high relief are located in the spandrels on the ground floor. With the exception of the arch in correspondence to the entry of the library which has Neptune holding a trident and Aeolus with a wind-filled sail, these figures represent allegories of non-specific rivers, characterized by the cornucopias and the urns with water flowing out. The enlarged keystones of the arches on the ground floor alternate between lions’ heads and the heads of pagan divinities (Ceres?, Pan, Apollo, Diana, Mercury, Minerva, Venus, Mars, Juno?, Jupiter, Saturn, and Phanes).