Maroon-coloured, thickly velvet-textured flowers with very broad petals, the tips of which are shaped like the end of a Canadian canoe. The petal colour, shape and texture set this aside from the related erectum, but the two are instantly separable, once you have seen them both side by side. The purple-grey pollen of sulcatum will confirm the identity (yellow pollen in erectum).
For the gardener this is effectively a very superior deeply coloured Trillium erectum, and indeed it sometimes masquerades as that in the trade.
Now a very scarce item, these are lovely mature divisions of a long cultivated stock.