Hylocereus ID and question

[rrozzi]

Here's a vigorous Hylocereus. I've always ignored H. undatus as an option because the ribs have always been pretty linear and had no curves/undulations. However these new large arms are forming with prominent rib undulations and a more prominent thorn at the center of each areole. (Note - the parent was the base of one of those grafted red Gymnocalycium things they sell at hardware stores).

Another question: In reference to the last picture, I've seen this before where a rib forms at the base or mid-stem and then terminates mid-stem. Is there a name for this phenomenon?

Thanks all!

[teo]

Looks like H. undatus to me, also that is the most common as grafting stock.