Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Diamante citron
The Diamante citron ("Citrus medica" var. "vulgaris" or cv. "diamante" − , or גינובה) is a variety of citron named after the town of Diamante, located in the province of Cosenza, Calabria, on the south-western coast of Italy, which is its most known cultivation point. This is why this variety is sometimes called the "Calabria Esrog". ""Esrog"" is the Ashkenazi transliteration of the Hebrew name for citron.
The Diamante citron was one of the most important varieties candied by the largest factories at Livorno, Italy; it was gathered from Liguria, Naples, Calabria & Sicily and then shipped into England and the United States.