10 Aug 2009 e-mail from Margriet Wetherwax, including forwarded information from Dieter H. Wilken, author of Ceanothus for The Jepson Manual [Ed. 2] see Comment 1
Correspondence 1 indicates that, according to Wilken, Ceanothus X vanrensselaeri Roof is a "putative hybrid described by Roof as C. incanus X C. thyrsiflorus".