This is interesting, I didn't originally even realize that there were other subspecies than Thamnophis marcianus marcianus, though in hindsight the need for the doubling up of the ssp name tipped it off.

From what I can find so far it appears of the 2 non US ssp, that bovalli is found SW of Nicaragua into Northern Costa Rica, while praeoccularis is found in the Yucatan Peninsula (Mexico), Belize, parts of Guatemala and into Honduras. I haven't yet found a good source for any physical differences.

Don, that's one of the books on my wish-list. If anyone has a copy and can pass along what the physical differences might be that would be very cool.