This association is restricted to the Pacific base of the range, is typical on the alluvium at the bases of the coastal foothills, and usually grades into the chaparral at about 1800 feet elevation. When seen from above, the rather level terrain of the association is broken sharply at the mouths of canyons by dry washes, and is limited below, to the south, by cultivated land. The coastal sagebrush is the most characteristic plant of this association, occurring in all undisturbed parts of the area. There are several habitats within the coastal sage scrub association. These differ from one another chiefly on the basis of soil type. The soil of the rather level sageland in most places is rocky or gravelly, or, as adjacent to washes, it is finely sandy in texture, and supports the major plants of the association. Most of the eroded adobe banks at the bases of the foothills support these same plants, with white sage being the dominant species. Locally, as in damp hollows or cleared areas, there is grassland. Jumbles of boulders, sand, gravel, and steep cutbanks, are characteristic of the channels of dry washes, these areas supporting sparse vegetation. The fauna and flora of the washes are distinct from those of surrounding sage flats. Because they are included within the geographic limits of the coastal sage belt, however, the washes are discussed along with this association. The abruptness with which one habitat gives way to another in this association causes sharp dividing lines between the local ranges of certain mammals. For example, in trap lines transecting dry washes and level sageland two assemblages of rodents were found. That part of the line amid the boulders and cutbanks of the wash took mostly _Peromyscus eremicus fraterculus_ and _Neotoma lepida intermedia_, while _Perognathus fallax fallax_, _Dipodomys agilis agilis_, and _Peromyscus maniculatus gambeli_ were taken in the adjacent sage flats. The steep adobe slopes of the foothills, which constitute the upper part of the coastal sage scrub association, are commonly inhabited by _Peromyscus californicus insignis_, which rarely occurs in the level tracts of sage a few yards away. Thus, this association is not homogeneous with regard to its rodent population; many of these species have local and discontinuous distributions. The following list gives the results of about 500 trap nights (a trap night equals one trap set out for one night) in typical coastal sage-scrub association one-half mile southwest of the mouth of San Antonio Canyon, at 1700 feet elevation. TABLE 2.--YIELD OF 500 TRAP-NIGHTS IN THE COASTAL SAGE SCRUB ASSOCIATION.