This is one I found this morning on the coast. Under the microscope the leaves look distinctive having the shape similar to a Zygodon with the costa being in a channel. I eventually identified it as either Didymodon luridus or Didymodon tophaceus. It was a bit of a tough call but eventually I went with Didymodon tophaceus because of the leaf shape and the habit of the leaves when dry as D.luridus has appressed leaves when dry and D.tophaceus has it leaves incurled and slightly twisted.
An expert bryologist ID'd a plant for me as being C.crassinervium yesterday but upon checking it the next day I couldn't make out the dinner plate shaped leaves and the basal half of the costa was not thick. Running it through the key I notice that Cirriphyllum has strongly papillose setae which mine didn't plus the lid was subulate and not rostrate. Eventually the key ran me to Rhynchostegium and it quickly became apparent that my specimen was in fact Rhynchostegium confertum.
Growing on a stone on a wooded slope made Didymodon vinealis a suitable candidate for this one. The lower leaves were darker than the upper and when dry they looked appressed to the stem. Under the microscope the cells over the costa in the upper part of the leaf were quadrate.
Here's a species that is potentially overlooked and at first site you could be forgiven for thinking this is Dicranella heteromalla. It's not mentioned in the book as a similar species but that's exactly what it was called by an expert bryologist until he got it under the microscope. A distinguishing feature is the deciduous shoots but when absent it can hamper a safe ID. Under the microscope the basal cells will help separate D.denudatum from Dicranella heteromalla.
This was an interesting one as this sample was identified in the field by a professional bryologist as Cirriphyllum crassinervium so in theory it should have been straight forward to key out. However, after running through the key it became apparent that it doesn't key to Cirriphyllum crassinervium. The leaf tip was the first problem being obtuse and not abruptly contracting to leave a short apiculus. This avenue led me then to chose between Rhynchostegium and Scleropodium. The key says that Scleropodium has differing sizes in stem and branch leaves whereas Rhynchostegium has them all the same size. I could see varying leaf sizes so opted for Scleropodium. This group only has two species which can be separated by habit when wet and dry plus S.tourettii has much more concave leaves than S.cespitans looking more like Pseudoscleropodium purum. This species can be eliminated by the long pinnate growth unlike the bushy matted like habit of S.tourettii. So we have an ID but look again the the thick costa in the basal half, this is typical of C.crassinervium and with the concave leaves with the flattened border we find out that it is C.crassinervium after all. As a postscript it is worth mentioning that Rhynchostegium murale is a similar looking plant and should be considered when identifying in the field perhaps.