Hooper, 1938 Berkeley, Alameda Co., Calif. Mar. 7, were abundant. One was seen running through the vegetation. About 1 mi. NW of this locality in dense, high Salicornia (~2 1/2 ft. high) along the railroad tracks to Liberon Penins., I put out 44 mouse traps about 8 ft. apart. Found Sunday: 3 Microtus, 2 Reithrodonys, and 2 Mus. The latter 2 were discarded. This locality = salt marsh, 1 mi. ESE Corte Madera. As a result of this trapping it is apparent: (1) That Reithrodontomys raviventris occurs north of S.F. Bay. (2) That its presence here must be explained. (3) That its range may extend on northward to that of R. thalictes. (4) That R. raviventris and R. megalotis occur together where the grassy slopes meet Salicornia. (5) That trapping must be done from San Rafael northward to clear up some of these matters. The fact that R. raviventris doesn't occur out of the Salicornia would seem to me to indicate that it is a native kind to this habitat which has spread here from the south or that it has been intro- duced into an unoccupied niche or displaced heterote.