Stumbled across this, You might find it interesting but also an older post. My step 2x great grandmother was a de St. Croix. In my research i have found things you probably know. St Croix used to be a city in french canada. De St Croix could mean that the family was (directly translates) "of St. Croix". The other less likely possibility is that they were decedents of french clergy members. The best I can say without being confusing is that "Du/De S. Croix" Literally means. "Of the Holy Cross"