I did some looking for info on what we call "yams".  My old hard copy
version of the USDA food list shows them as Impomoea species (spelling?).
  Joy of Cooking, Encyclopedia of American Cooking, and others all say
that what we commonly call a yam is a variety of sweet potato,
distinguished by a softer, moister and sometimes darker flesh.  True yams
show up (in my limited library) as a Japanese tuber, different textured,
much longer, darker skinned and white fleshed.