yes it's a real mess with the sessile flowering Trilliums.
i thought i grow
Trillium kurabayshii in my garden, have a lot of plants from different sources. their shape is very uniform. but when we had a discussion about identifying chloropetalum/kurabayashii in a german garden forum, i checked the
Trillium key in Flora of North America to find a significant, but easy to use characteristic to distinguish these 2 species...i guess i found it...it's the proportion of stamen to carpel and the length of the connective (the tissue between the pollen sacs). pics show more than words can explain, so i made a collage to show the differences. the
chloropetalum flower detail is from one of my garden plants, the
kurabayshii foto is from internet, token in the wild in oregon (to be shure, that it is not mistaken with
chloropetalum). the pointers are in german, but you will know, what i want to say + for what you have looking for....hope this helps. any hint is welcome when a mistake is found.