Specimens were then examined in a shallow dish containing glycerin under a binocular microscope. The baculum can be viewed from any desired direction. The method described above leaves the baculum intact within the glans penis; therefore its orientation can be determined relative to the thick walled urethra and the thin walled dorsal artery that extends onto the dorsal side of the baculum. The ventral curvature of the penis proximal to the baculum, and the distal extension, characteristic of most species, of the dorsal border of the glans (both shown in Figure 1) are other features aiding in correctly orienting cleared specimens. The digitate processes are not so often injured, lost, or displaced when the method described above is used as they are when the penis is dissected. Specimens were stored in glycerin in glass shell vials having polyethylene stoppers. A small card bearing the name, number, locality, and other data was placed in each vial. A specimen thus enclosed can be kept indefinitely, or removed and mounted in balsam as described by White (1951:631) or in plastic as described by Dearden (1958:541) and thus stored in the vial containing the skull of the specimen. Drawings were made on millimeter ruled paper while the baculum was viewed under a binocular microscope with a square ruled eyepiece. Unless otherwise noted all specimens listed are in the University of Kansas Museum of Natural History. Catalogue numbers are cited. Measurements are accurate to within less than one-tenth of a millimeter. Proportions as stated in the text are approximations, accurate to within one-twelfth (8.33 per cent). The range of variation is unknown for some species. Mention is made if maturity is known or suspected to differ in specimens being compared. The development of the baculum has been studied by Callery (1951) in _Mesocricetus auratus_ and by Ruth (1934) in the laboratory rat. In the rat (_Rattus norvegicus_) the bone is of endoblastemal origin being laid down by a condensation of undifferentiated mesenchymal cells. At the distal end of the bone dense fibrous tissue is then differentiated and at the proximal end hyaline cartilage. Growth is by substitution at the proximal end and by subperiosteal lamellation circumferentially. A marrow cavity is formed by resorption. In the baculum of the hamster the primary center of ossification is in the stalk, and is present at the age of three days; the secondary centers are in lateral processes and are present at 80 days and enlarge subsequently. A tertiary center, in each median process, may or may not develop later. Maximum development of the baculum is reached late in the reproductive life of the hamster.