I have Dioscorea mexicana (= D. macrostachya), a single plant, and a bunch of seedlings of D. sylvatica. D. sylvatica seems to be a very variable species, and some forms adopt the "Turtle shell" appearance of D. mexicana and D. elephantipes. Some are more irregular. The seller of the D. sylvatica seeds (Seedhunt, plus Seedhunt's source, whom I know) claims that it is much faster growing than D. elephatipes and D. mexicana. I think they are right. I got quarter-size caudexes/tubers from seeds planted last June, smaller ones planted later (September?). I would strongly recommend this species (and this source) if anyone wants to try. Their seeds are cheap, they send more than they claim, and germination is quick and very high. The only drawback is that the caudexes might (?) end up more irregular shaped than the "classic" shape. I also have seedlings to trade, if anyone is interested, but honestly, the Seedhunt seeds are so phenomenal I would recommend them first. $4.50 for 50 plants, quickly, is not bad. Here's a link:
http://www.seedhunt.com/pplist.html
Here's a seedling of D. sylvatica, in November, that was started in June:
That said, I would still recommend D. elephantipes and D. mexicana if one wants to be sure to get the "tortoise shell" shape, as I don't know what this variety of D. sylvatica will end up looking like. However, they are expensive plants (my D. mexicana was an incredibly generous gift) and are slow growers.
I also once had a 6 inch caudex/tuber of D. mexicana stolen by squirrels when I lived in Los Angeles. It is a yam, and they decided it was food...