The Northern Pocket Gopher (Thomomyst alpoides) is common on open range lands at high altitudes, and range management personnel have long been confronted with the problem of controlling gopher populations. Although some studies indicate that pocket gophers have little or no injurious effects on range in good condition, other studies show that large populations of these animals can seriously damage seeded ranges and ranges in poor condition (Colorado State University Exp, Stat., 1960). Julander, Low, and Morris (1969) indicate that in areas where gophers have reached populations of 27-39 gophers per acre, forage removal by gophers may be from 4.75 to 7 pounds of fresh weight vegetation per acre per day. This converts to 435-670 pounds of airdry plant material per acre per year. On depleted ranges this represents a large percentage of the total annual growth. Hansen (1965) reported that in 1961 gophers numbered 52 per acre on Black Mesa, Colorado, Such a high density of gophers could have drastic effects on range soils and vegetation.

The purpose of this study was to determine the morphological differences in the osteology and musculature of the thoracic limb of the pocket gopher and kangaroo rat. This was accomplished by first, collection of specimens, second, the preservation and preparation of bones and muscles tor study and illustration, third, sketching and photograph­ing the sketches, fourth, noting and recording origins and insertions as well as any differences in the modification of anatomy with relation to habitat requirements. These two closely related families, Geomyide and Heteromyidae, show great morphological differences. Comparative volumetric measurements show that the forelimb of the pocket gopher is approximately twice the size of the fore­limb of the kangaroo rat. Differences in the myology were found in relative length, thickness, shape, overall size, and in origin and insertion. Variations were also found in the osteology of the two forms. These differences were gen­eral outline, locations of the tuberosities for muscle attach­ment and in the length and placement of the various bone processes.

The purpose of this study was to compare the morphology of the hind limb of the pocket gopher with that of the kangaroo rat and to determine their differences. This was accomplished by first, obtaining the specimens; second, preparation of the specimens for preservation and dissection; third, photographing and tracing the muscles; fourth, noting the origin and insertion of each muscle and their general morphology; and fifth, preparation and photographing of the bones. Comparative volumetric measurements were made of the muscles of the two animals to determine the differences between them. These two families, Geomyidae and Heteromyidae, are closely related but show great morphological differences which are adaptive due to their different environments and habits. The volumetric measurements of the two animals showed the shank of the kangaroo rat to be twice the size of that of the pocket gopher. The muscles used by the kangaroo rat in jumping are longer and larger as indicated by the volumetric displacement and linear measurements. Also, linear measurements were made of the various muscles and bones, These measurements also showed the variation in lenth and width, with those muscles of the kangaroo rat baing longer and wider. In the study of the bones, the general outlines of the two forms were vastly different. The location of tuberosities, size of the tuberosities for muscle attachment, bone length and width made up these differences. Origins, insertion, actions and description of the muscles are given for each animal.