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On September 12, 1941, the writer accompanied Park Naturalist George C. Ruhle to the Lava Beds National Monument which was at the time associated for administrative purposes with Crater Lake National Park. Fifty live-traps were set out in the vicinity of Tule Lake and 32 adultPeromyscus maniculatus were taken. These were measured and skilled and the pelages added to the study skin collection at Crater Lake National Park. A statistical study of the measurements is being recorded here for the reference of students of the distribution and systematics of small mammals.

Methods: All measurements were taken of recently anaesthetized specimens removed from the live traps. The mouse is stretched to a uniform tension on a measuring board and the dimensions of the total length, tail length and foot length to the end of the longest claw are recorded with pins. The dimensions are then taken with a vernier caliper. The ear is measured, with the same instrument, from notch to tip. Body length is obtained by subtracting the tail length from the total length. The tail percentage is then obtained by dividing the tail length x 100 by the body length. It allows a better comparison of mice of different size than actual tail length. Tests have shown that this method of measuring recently anaesthetized mice can be done with an average error not greater than one-tenth of a millimeter.

Body Length: This standard measurement varies with age. All specimens included were 85 mm or more in length which is taken as the lower limit of adult body length. This allows comparison with series from other places with a similar lower limit but does not, of course, entirely eliminate differences in average size due to age differences in populations. The arrays are tabulated below:

In Peromyscus, as in man, female mice have a better life expectation than males. Female adults in the Lava Beds are older and therefore bigger than males. In this series they are also more numerous.