Romana Lemon or Lima dulcis
This is a low acid ancient lemon that you can see in some old Italian paintings.
It is catalogue number 333 in Ferrari's "Hesperides"
It is almost certainly a limetta like the Palestine Sweet Lime.
Nearly round except for it's very pointy nipple,which looks a bit like its been pushed in, the fruit is sweet but a little insipid without the acidity but the kids love the flavour.
A definite must have for collectors of old Citrus varieties.