108 



NORTH AMEEICAN FAUNA. 



[no. 39. 



smoothly rounded, with anterior arm of basioccipital slender and 

 narrow between them. Dentition: Upper incisors sharply incurved 

 so that the cranium can be readily suspended by them, and lower 

 incisors also curved very abruptly upward. 



Measurements. — Type (a large old male): Total length, 204; tail 

 vertebrae, 60; hind foot, 26. Topotype ( c? ad.) : 188; 51, 24. Aver- 

 age of 6 topotypes ( ? ad.) : 194, 58, 24.6. Ear: From crown, meas- 

 ured in flesh, 5. . Slcull (of type): Basal length, 33; nasals, 12; zygo- 

 matic breadth, 20; mastoid breadth, 17; interorbital breadth, 5.5; 

 alveolar length of upper molar series, 7. 



Remarlcs. — This is a little, pale, desert species of the Green River 

 Basin, mainly in the Upper Sonoran Zone. It occupies the dry, sage- 

 brush mesas, while hridgeri occupies the fertile valleys close by. 

 Fort Bridger seems to be the extreme upper limit of ocius, and here 

 its range meets that of the much larger hridgeri and almost or quite 

 meets that of the smaller, darker-colored pygmseus. On the east it 

 meets the range of clusius, but I find no signs of intergradation, and 

 the skull characters are so stronglv marked that it seems best to 

 treat ocius as a full species. Its nearest relatives are idahoensis of 

 southern Idaho, and more remotely the little pygmseus, along its 

 western border in the Transition Zone. These three forms are the 

 only members of the genus in which the crania can be readily sus- 

 pended by hooking the upper incisors over a wire or string. The 

 ears of ocius are very small, measuring uniformly 5 mm. from pos- 

 terior base to tip in fresh specimens, while in hridgeri from the same 

 locahty they measure 8 mm. The females have normally 14 mammae 

 and contained 7 fetuses, contrasted with 10 mammse and 5 fetuses 

 in hridgeri, collected at the same time and place. 



Specimens examined. — Total number, 38, as follows: 



Colorado: Douglas Spring, 1; Ladore, 2; Lay, 1; Lily, 2; Raiigely, 2; Snake 



River (20 miles west of Baggs), 2. 

 Utah: Uncompahgre Indian Reservation, 2. 



Wyoming: Bitter Creek, 2; Eden, 1; Fontenelle, 1; Fort Bridger, 9; Green 

 River (junction of New Fork), 3; Henr^^s Fork, 1; Maxon (5 miles south- 

 west), 8; Opal, 1. 



THOMOMYS IDAHOENSIS Merriam. 

 Idaho Pocket Gopher. 

 (PI. II, fig. 7; PI. VII, fig. 10.) 

 Thomomys idahoensis Merriam, Proc. Biol. Soc. Washington, XIV, 114, July 19, 1901. 



Type. — Collected at Birch Creek (10 miles south of Nicholia, at about 

 6;400 feet altitude), Fremont County, Idaho, by Clark P. Streator, 

 August 8, 1890. Type specimen in U. S. Nat. Mus., Biological Sur- 

 vey collection. 



Distribution. — Snake River Plains, southeastern Idaho (fig. 10). 



