DAKOTA AND NEBRASKA. 



337 



mys monax, or of the Squirrel, Sciurus caroUnensis, and mainly differ in the distinct 

 development of a pair of lobes to the crown internallj-, instead of one as in the former 

 animals. 



As before intimated, the crowns of the lower molars resemble those of the upper 

 ones in a reversed position. 



In the second upper molar, or premolar, the bilobed character of the inner part of 

 the crown is less distinct than in the true molars, in consequence of feeble develop- 

 ment in the postero-internal lobe. 



The moderately worn triturating surfaces of the upper molars, except the first or 

 small one, in the fossil skull, exhibit crescentoid tracts upon the summits of the inner 

 lobes, with transverse prolongations from their concavities along the summits of the 

 outer lobes, and narrow prolongations outwardly from their distal horns. The sj^eci- 

 mens of inferior molars exhibit similar worn surfaces in a reversed position. 



The measurements of the specimens are as follow 



Distance from inion to fronto-nasal suture, . 



Width of cranium at the root of the zygomatic process, 



"Width where narrowest, in the frontal region, 



"Width of forehead at fronto-maxillary suture. 



Length of sagittal crest, 



Length of parietals along the latter, 



Length of frontals along the middle, 



Distance from posterior nares to incisive foramina. 



Breadth of hard palate between middle molars, 



Length of superior molar series, 



Antero-posterior diameter of third molar, 



Transverse " " " 



Antero-posterior diameter of second lower molar, 



Transverse " " " 



Lines. 



21J 

 Hi 



4i 

 10 

 16 



7 



9i 



9 



3i 



7i 



2 



li 



2 



If 



Dr. Hayden's last collection of Mauvaises Terres fossils contains a number of frag- 

 ments of jaws o? Ischyromys typus, which are the first specimens I have seen from 

 that locality, the previous ones having been derived from Bear Creek. Among them 

 are the greater portions of four halves of lower jaws, all containing full series of 

 molars, while others of the upper and lower jaw contain from two to four. These 

 afford us an opportunity of rendering our knowledge of the permanent dentition of 

 the animal complete. 



One of the best preserved specimens of the lower jaw, represented in figure 3, plate 

 XXVI, nearly agrees in form with the corresponding portion in the Squirrel, but the 



43 



