R. rhombea
We all grow rhombea but Barthlott & Taylor (Bradleya 13) says "This name cannot be typified and in herbaria, the literature and living collections it has been, and continues to be used in a multitude of senses, including R. crispata, R. occidentalis, R. pachyptera, R. elliptica, and R. oblonga. Its use should be abandoned and ideally it should be formally rejected under the provisions of the International Code." 

So where should it be placed? Until a final decision is made about this so-called species, we'll include the information and photographs we run across.

Flat-leafed species with wavy or notched edges that can be reddish if grown in strong sunlight. Leaves can be three-angled. Prominant mid rib and noticeable lateral veins. Pale yellow flowers and red fruit. Sao Paulo.

Photo taken at Huntington Botanical Garden 9/99, KAF.