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These comparisons are made with a sample of 72 mice from a sage brush association in Silver Lake, Oregon, a sample of 45 mice from Crater Lake South Rim and a sample of 27 mice from a yellow pine and deer brush association at the South Entrance to Crater Lake National Park. Lava Beds mice do not differ statistically in body length from any of these other samples.

The tail percentage is usually bound to be smaller in larger mice. These females averaging larger than the males have relatively shorter tails.

It has been found repeatedly that the relative tail length in Peromyscus varies directly with the amount of precipitation and therefore of vegetation covering the ground. Tails are longer where ground cover is more abundant. The Lava Beds are the most arid of the habitats in which series have been taken. They differ in relative tail length slightly from the South Entrance series and considerably from the Crater Lake Rim series the differences being 5.9 +/- .95 per cent. This difference is six times its standard error and therefore statistically significant.

Females have slightly longer ears presumably because they are larger mice.