The genus is
There are two subgenera within this genus: Ceanothus and Cerastes. The former clade is less drought-resistant, having bigger leaves. The evolution of these two clades likely started with a divergence in the niches filled in local communities, rather than a divergence on the basis of geography.
The Californian species of Ceanothus are commonly known collectively as California lilacs, with individual species having more descriptive common names. Species native elsewhere have other common names, such as 'New Jersey tea' for C. americanus, since its leaves were used as a black tea substitute during the American Revolution. In garden use, most are simply called by their scientific names or an adaptation of the scientific name, such as 'Maritime ceanothus' for C. maritimus.