Omphalina mushrooms are very small and some species grow only with lichens. These three are growing on a soggy fallen tree trunk. Other very similar looking mushrooms are in the genera Mycenas and Clitocybes. "Mushrooms Demystified" and the smaller field guide by David Arora are really helpful references. However microscopic examinations, spore prints and a lot of experience are necessary to get beyond the beginner stage.