Sorry, to be more precise:
C. Lemon 'Lipo'(Imperial Lemon) is belived to be C. Limon x C. paradisi.
C. lemon Ponderosa is believed to be Citrus limon × medica.
Both plants are similar regarding leaves, grow habit, blossoms just the fruit taste is different, Lipo has poor lemon aroma.
I've also C. Lemon nine pounder(no fruit yet) which is very similar with Ponderosa and Imperial Lemon just has few nice thorns.