Synaptomys cooperi Baird
Figs. 5 and 6
Baculum: Stalk elongate, greatest length (2.7 to 2.8 mm.) 2 1/3 to 2½ times greatest breadth, 4 to 5 times greatest depth; three processes ossified or lateral processes unossified, ossifications relatively small (in 78380, median ossification less than ¼ as large as lateral ossifications although median cartilaginous process is larger), length of median process 1/5 to 1/6 of length of stalk, cartilaginous part of median process larger; posterior profile in dorsal view convex throughout or bilobate; tuberosities moderately developed, deflected dorsal to axis of shaft; in end-view medial construction 3/5 greatest depth of tuberosities; shaft tapered from point of greatest width, slightly inflated terminally.
The specimen (KU 13716) figured by Hibbard and Rinker (1942:29) has been restudied. It was first cleared and stained to soften the dry cartilage binding the digital processes together and to differentiate bone and cartilage. The lateral processes are small and cartilaginous (Fig. 6) and seem intact. The differences between this specimen and others examined by Hamilton (1946:381), Dearden (1958:542), and myself, namely the relatively larger median ossification, the absence of ossification in lateral processes, and the distinctly bilobate base and larger size, may represent geographic differences, or individual variation. The proportions of length, width, and depth of the stalk, and the appearance in lateral view do not differ greatly from others examined by Hamilton, by Dearden (1958:546), and by me.
Specimens examined: Five, representing four subspecies; S. cooperi gossii, 6 mi. N Midway, Holt Co., Nebraska 78379, 78380; S. cooperi relictus, 5 mi. N, 2 mi. W Parks, Dundy Co., Nebraska, 72601 (immature); S. cooperi saturatus, 3 mi. S Demotte, Jasper Co., Indiana, 3-C-454, collection of W. B. Quay; S. cooperi paludis, Meade County State Park, Kansas, 13716.