The most current list of mammal species of special concern may be found on CDFW's 
Special Animals List (PDF).
In 1986, the Department of Fish and Game published 
Mammalian Species of Special Concern in California (PDF) by D.F. Williams. The 1986 publication included a list of 36 species and subspecies of land mammals native to California determined by the author and the Department to be potentially threatened with extinction in the State. Species accounts for each taxon included initial description references, information on distribution, population status, habitat, recommendations for additional assessment and conservation actions, taxonomic remarks, and distribution records.
A 1998 Draft Update (PDF) contains valuable information and maps, but was never finalized by the contracted writers and did not receive peer review.
CDFW is in the final stages of a new update to the MSSC list using the criteria and methods outlined on the main Species of Special Concern web page. A team of California mammal experts, led by Conservation Biology Institute (CBI) and CDFW, assessed the conservation status of all California's native terrestrial mammals. A draft proposed list of MSSC has been prepared and is currently in internal review, along with draft species accounts and other information. The list will be made public in the near future.