I can't find my Smith's Liverworts of Britain and Ireland and haven't looked at it since Jean Paton published her Liverwort Flora in 1999 - Jean's book seems to costs over £90 now, but it is worth every single penny. I am pretty sure that early on in Smith's Riccia key the split is whether your specimen looks spongy or not - back in about 1994 I was sure the first Riccia I had come across in Carmarthenshire was R. sorocarpa, and to me it looked 'spongy' ....... I followed the key and think eventually ended up at R. cavernosa - this was clearly wrong and then it occurred to me that the reference to sponge probably meant natural sponge, which is probably unfamiliar to a lot of people today and was a bit of a rarity even when I was a kid in the 1960's (or perhaps it was expensive and we could only afford artificial sponge - actually we had a loofa which tended to take your skin off if you rubbed too hard!) - the only times I seem to see natural sponge these days is in art supply shops (usually as expensive tiny fragments).
Anyway, whilst at Llangorse Lake last week I glanced down and saw what I first took to be a thick layer of algae amongst open vegetation in the inundation zone (the lake level at Llangorse tends to be a lot higher in winter than in the summer months). On closer examination the 'algae' turned out to be the largest population I have ever seen of what looked to be R. cavernosa. I had it in the back of my mind that Sam had recorded cavernosa at Llangorse when he mapped the marginal vegetation several years ago, but on reading his report he in fact had found R. subbifurca and, if Sam accepts the voucher, cavernosa will be a new addition to the Brecks bryophyte flora.
Pics below are a bit poor, but the close-up shows the large perforations on the surface - a bit like an old fashioned sponge.