474 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



a small median lobe. Jn tlie last premolar, the conic cusps are well 

 separated, and the inner one of the heel is insignificant. This tooth 

 appears to have been the last one protruded ; its temporary predecessor 

 is distinguished by the obtuseness of the cusps, especially of the ante- 

 rior one. Mandibular ramus deep, compressed, without inferior hook 

 as far as oi)posite the basis of the coronoid process. 



Measurements. 



M. 



Length of sectorial and two tuberculars 0.0210 



Length of sectorial alone 0045 



Width of sectorial 0020 



Width of first tubercular 0030 



Length of first tubercular 0032 



Depth of jaw at first tubercular 0060 



Size of a skunk. 



E O D B NT I A . 



Species of this order are numerous in the Tertiary of Colorado, and 

 the individuals were more abundant than those of any other type of 

 mammalia. Hundreds of specimens of some of the species were found, 

 which range from the size of a marmot to even less than the domestic 

 mouse. The relationships of these are as follows : 



Muridce : 



Uimiys, Leidy 1 



Sciuridaj : 



Sciurus, Linn 1 



Gymnoptyolms, Cope 2 



Incert^e sedis : 



MeUscomys, Cope 1 



Ischyromys, Leidy 1 



Leporidse : 



Falceolagus, Leidy 4 



10 



BDMYS, Leidy. 

 31us {Mimys), Leidy. 



The single species embraced in this genus is nearly allied in dental 

 and the known portions of its cranial characters to tliose of the existing 

 genus 3Ius. The only distinctive feature which I can discover is that 

 the supraorbital ridges rise from the lachrymal bones, and unite, forming 

 a median keel between the orbits as in the Fiber. Whether it is found 

 in any of the numerous existing species of ilfws, I cannot state ; but the 

 frontis plane and the superciliary ridges are well separated in Mus 

 decunianus. The foramen infraorhiiale anterius is much as in Mtis 

 decumatius, being large and continued into a fissure below. The denti- 

 tion is also similar, including the composition of the molars. These 

 support two rows of obtuse tubercles, the number increasing with the 

 size of the teeth from behind forward. Formula: L, i ; C., -^-j M., f. 



EuMYS ELEGANS, Leidy, Ext. Fauna Dakota and Nebraska, p. 342, PI. 



xxvi, Figs. 12-13. 



This species is exceedingly abundant in the Colorado Miocene, and 

 many specimens were obtained. These display considerable variation 

 in size and robustness j some, perhaps males, having the muzzles stouter 



