I often hear a similar mistake - “vertice” for “vertex”. I think you do give too much credit by hypothesizing a borrowing from French. They’re just treating “indices” or “vertices” as an English plural and back-forming a singular. By the way, the pronunciation of “indice” in French is closer to “ann-deese” than “on-deese”.

Well, Ken, I just want you to know that I thought of you yesterday when I was giving a talk in front of a large group of attorneys. At one point, I alluded to the obitofrontal cortex of the brain, then heard myself blurt, “In people whose orbitofrontal cortices have been damaged — oops, is it cortexes or cortices?” and then stumbled a bit, realizing I didn’t know. So thanks a lot, Ken!

Looks like it can be either one. And John, I agree it’s quite possible that it’s just a natural formation by analogy.

I’d be stumbling on “orbitofrontal.”

Believe me, I did! So of course I got completely tangled up with “orbitofrontal cortices”!

Yes, I actually had a teacher in high school who would ALWAYS refer to several matrices, or one matrice! Pronounced similarly to “indice” that the caller referred to.