Extensive fields of mammatus dangle beneath the undulating underbelly of an anvil canopy, over the Texas Panhandle. As far as I've found, there has been no specific cloud categorization called mammatus undulatus. Consider it done, right here, right now. It is yet another example of how the long-established naming system for clouds just doesn't accommodate hybrid forms and strange processes like this. If this isn't be a textbook example of something that could be called "mammatus undulatus," I don't know what is.
2 SE Dalhart TX (10 Jun 5) looking NE