Here is a picture of a citron from googling images for citron.
http://www.tunedal.com/images/citron.jpg

It looks quite similar to a lemon AFAIC.
***

On p. 66 of Bugialli's book "Classic Techniques" two male hands are shown
slicing up "a whole glaceed citron." Looks like something pretty big and it
says,
"Citron resembles a large lemon but is a different fruit in that it is
larger, shaped like a quince, and has a rough thick skin. It is very
popular in the Mediterranean and it's mostly used there and elsewhere for
making glaceed fruit."

Here is another link that has two pictures, both quite different
http://www.hort.purdue.edu/newcrop/morton/citron.html

Has anyone seen citron in any markets (here in the U.S.)?
Thanks,
Dee