So, I've got a spider that looks like a female Xysticus cristatus, and that's dead common so it's probably what it is. However, I'm trying to rule out the other species properly. I think I can rule out most except X kochi and X ulmi. But to clinch the deal, I apparently need to make reference to the epigynes.

Unusually, I can see the epigyne on this one pretty well (even with it still being alive it's quite cooperative). But I'm pretty confused about what the differences are. I've got the Collins spider guide by Roberts, which has drawings and helpfully tells me that the epigyne of both kochi and ulmi are "distinctive". Hmm. I've also googled images of the epigynes of all three, and am still pretty flummoxed on what the differences are.

As far as I can see, there's a pair of holes, with a column-like thingie separating them. Not sure what else I should be looking at. 

Does anyone have advice? Perhaps there's a handy online guide somewhere?

Thanks as always

Andy