San Joaquin R., 3 mi. N.E. Vernolis, San Joaquin Co., Calif. October 19. Providing some sort of continuity to the trap line. At least 15 wood rat nests were found, all with fresh sign about them. Though most of the nests are in oak trees (Quercus lobata), some were found on willow stumps. They were composed of dry, water-worn sticks up to 4 ft. in length x 2 in. in diameter, were not over 2 ft. in height and were placed at heights above the ground varying from 3 feet to 20 feet. We so disturbed one nest in an oak tree that 2 rats [1 juv. ♀] and 1 ♂ with 4 conspicuous inguinal mammas ran out. One was shot, the other (? perhaps) was trapped last night by D. H. Johnson. The rats here are distinctly inclined toward arboreal habits, more so, apparently than other members of the Neotoma group. No nests have been seen on the ground, though burrows at bases of trees harboring nests were seen. Is this an extreme adaptation of a tree-inhabiting characteristic of the Neotoma group, made extreme because of the annual flooding? Perhaps selection has been active in weeding out the non-tree climbers.? At any rate there are certainly many more rats here than I expected, and thus are apparently more nests than were her