1914.] 



EEITHEODOKTOMYS MEGALOTIS GROUP. 



25 



Slcull. — Similar in shape to that of R. m. megalotis, but smaller and 

 braincase relatively narrower; zygomata (in adults) with sides par- 

 allel. Compared with R. a. griseus: Longer and relatively narrower, 

 with longer rostrum. 



Measurements. — Average of 10 specimens (including adults and 

 subadults) from Medano Ranch, near Mosca, Colo.: Total length, 126 

 (118-139); tail vertebras, 58 (51-64) ; hind foot, 17 (16-17); ear from 

 notch, 11.2 (11-12). Skull: (See table, p. 81). 



Remarks. — This species, the second member of the genus to be 

 recognized, was described by Baird in 1855 from a single specimen, 

 and until within the last few years has remained practically un- 

 known. Efforts were made by the Biological Survey field party 

 in 1904 to secure specimens from the San Luis Valley and a single 

 immature individual was caught at Del Norte by Vernon Bailey. 

 In the autumn of 1907, Merritt Cary trapped for about 10 days at 

 Medano Ranch, 15 miles northeast of Mosca, and succeeded in se- 

 curing a series of 20 specimens, including several adults, which for 

 the first time made possible accurate comparison and characteriza- 

 tion of the species. Most of the specimens taken by Cary were caught 

 in a grassy weed-patch on a broad sand-ridge extending through the 

 meadows and perhaps 6 feet above their level. None was taken 

 in wet situations. 



Dr. J. A. Allen has determined the approximate type locality of 

 this species fiom a careful study of the records of Capt. Beckwith's 

 expedition, on which the type was secured.^ The type specimen 

 (No. 13 of the expedition) was collected in August, 1853, in the 

 upper end of the San Luis Valley, at some point between Fort Massa- 

 chusetts and Sahwatch Creek. The route of the expedition was 

 northward along the east side of the valley to a point a little north 

 of 38° latitude, and thence westward across the vaUey not far 

 from the present town of Saguache.^ The second camp of the party, 

 on the night of August 24, was on a small stream nearly opposite 

 to Roubideau's or Mosca Pass," about 21 miles from Fort Massa- 

 chusetts. The specimens collected by Cary in 1907 were taken 

 along the very creek on which the Beckwith party camped, now called 

 Medano Creek. The type may have been taken on this creek or at 

 one of the other camping places a few miles farther north. 



Even with a series of 20 topotypes, the relationships of this species 

 are not entirely clear. This uncertainty is due in part to the fact 

 that the topotype series includes only two adults, one of which has 

 the skull broken, and in part to the failure to find in the series a 

 single skull that agrees with that of the type. The latter, an adult 

 with moderately worn teeth, is decidedly smaller and has a much 



1 See BuU. Am. Mus. Nat. Hist., VII, 1895, pp. 124-125. 



2 See Expl. & Surv. Pac. R. R., II, pp. 41-45, and map accompanying Vol. XI. 



