There are many different possible lemon varieties or cultivars that these could be. Knowing where you are located can begin to narrow down the possible cultivars. However to get you started, I've provided links to a University of Hawaii Extension publication on lemon varieties most commonly found in Hawaii and an Extension publication from Purdue University listing lemon cultivars commonly found in the U.S.  Additionally, determining variety type can be challenging from just pictures of the fruit. If possible, please provide picture of the whole tree and some description of their growth habit.

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/F_N-25.pdf 
https://www.hort.purdue.edu/newcrop/morton/lemon.html