Italian to English Other Botany olive description

From the description of an olive cultivar, the Sicilian "Santagatese", to be precise:

"Drupa di grossezza superiore alla media, piuttosto allungata a forma di batacchio campanario, più stretta alla base e più larga nel ***terzo tropicale***, lievemente mammellonato."

At a guess, I would say simply "middle", as I can only imagine that the author is dividing up the fruit as if it were the Earth (with the terzo tropicale being the area between the Tropic of Cancer and the Tropic of Capricorn, presumably), but I may be well off target, so any help from an expert would be appreciated!