_Agropyron smithii_ _Artemisia_ sp. _Eriogonum umbellatum_ _Lupinus ammophilus_ _Penstemon linarioides_ _Phlox hoodii_ _Stipa comata_ Arachnid legs Stomach and intestinal contents of _P. truei_ contained the following food items: _Artemisia nova_ _Artemisia_ sp. _Penstemon_ cf. _barbatus_ _Penstemon_ cf. _linarioides_ _Poa fendleriana_ Arachnid legs _Eriogonum_ sp. _Gutierrezia sarothrae_ _Yucca_ sp. Chitin Feathers Many of the plants eaten by the mice had large numbers of crystals in the epidermis. Druses were the most abundant, but raphid crystals also were seen. Every slide contained at least one species of plant which contained druses. Such crystals are composed mostly of calcium oxalate (Esau, 1960:41). In Mesa Verde, families of plants having crystals include: Boraginaceae, Chenopodiaceae, Compositae, Cruciferae, Leguminosae, Liliaceae, Malvaceae, Ornargraceae, Rosaceae, and Saxifragaceae. Calcium oxalate is a highly insoluble compound and is innocuous if it passes through the gastro-intestinal tract without being absorbed. In rats of the genus _Neotoma_, some calcium oxalate passes through the intestines unchanged, but large amounts of calcium are absorbed through the intestine. The urine of pack rats is creamy in color and contains calcium carbonate. It is not understood how these rats metabolize the highly toxic oxalic acid, when converting calcium oxalate to calcium carbonate (Schmidt-Nielsen, 1964:147-148). Apparently calcium oxalate passes through the intestine unchanged in both species of _Peromyscus_, for their urine is clear and yellowish. Although both species of mice appear to prefer plants having soft leaves, some plants having coarse leaves also are eaten. Many of the slides contained isolated sclerids. The stomach contents of one individual of _P. truei_ contained a small fragment of the epidermis of _Yucca_. This fragment may have come from a young shoot. It is unlikely that _Peromyscus_ would eat the larger, coarser leaves of _Yucca_. Pinyon and juniper nuts were found in nests of all mice. Captive mice were especially fond of pinyon nuts, and these probably provide a substantial part of the diet of _Peromyscus_ in the autumn and early winter. The winter staple of _P. truei_ appears to be juniper seeds. Nesting sites of this mouse often could be located by the mounds of discarded seeds lying nearby.