Welcome Lyle. You may find this of interest. http://www.greenfingers.com.au/services/digging_deeper/citrus/Sweet_Oranges.htm If you read down to Joppa oranges this has been written... "The 'Joppa' tree makes a stiff tree with thornless branches, where as most other oranges are armed with at least some thorns" Hope this helps
However, even "thornless" varieties often develop thorns due to the influence of the rootstock, which almost certainly came from a thorny variety (since they tend to be hardier/stronger plants).