Peter Werner who is working on his masters thesis on the Strophariaceae of California doesn’t think this is really a Stropharia, but he hasn’t looked at it closely yet.
11/.7.2010: I had previously misreported this as a Leratiomyces. It is not, in fact, either Leratiomyces or a proper Stropharia, but part of the Stropharia semiglobata group, a clade within the Strophariaceae who’s taxonomic relationships are not entire clear. According to Clemençon & Rofler, S. luteonitens and S. umbonatescens are synonyms. That being the case, “Agaricus luteonitens” is the older species name and takes priority. An even older synonym, “Agaricus nitens” is treated as a synonym for Stropharia semiglobata, and hence, not valid for this species.
Reference: Heinz Clémençon and Urs Roffler. (2003). The pseudosclerotia of the agaric Stropharia luteonitens. Mycological Progress 2(3): 235–238. Available from: http://www.springerlink.com/content/q72q770w92w73721/
Some links… Not sure what to make of it all: