The accepted view is that the main developments which differentiated the early dromons from the liburnians, and that henceforth characterized Mediterranean galleys, were the adoption of a full deck ("katastrōma"), the abandonment of the rams on the bow in favor of an above-water spur, and the gradual introduction of lateen sails. The exact reasons for the abandonment of the ram (, ) are unclear. Depictions of upward-pointing beaks in the 4th-century "Vatican Vergil" manuscript may well illustrate that the ram had already been replaced by a spur in late Roman galleys. One possibility is that the change occurred because of the gradual evolution of the ancient shell-first mortise and tenon hull construction method, against which rams had been designed, into the skeleton-first method, which produced a stronger and more flexible hull, less susceptible to ram attacks. Certainly by the early 7th century, the ram's original function had been forgotten, if we judge by Isidore of Seville's comments that they were used to protect against collision with underwater rocks. As for the lateen sail, various authors have in the past suggested that it was introduced into the Mediterranean by the Arabs, possibly with an ultimate origin in India. However, the discovery of new depictions and literary references in recent decades has led scholars to antedate the appearance of the lateen sail in the Levant to the late Hellenistic or early Roman period. Not only the triangular, but also the quadrilateral version were known, used for centuries (mostly on smaller craft) in parallel with square sails.