DAKOTA AND NEBRASKA. 45 



molar. The inner side of the second upper true molar wore a broad grooved surface 

 externally upon the back part of the first lower true molar and the fore part of the 

 second lower true molar. The inner side of the principal cusp of the upper sectorial 

 molar wore a grooved surface externally upon the back part of the second lower true 

 molar and the fore part of the lower sectorial molar. 



The premolars in the process of wear were blunted, as usual in the carnivora, by 

 wearing from the apices of the constituent lobes of their crowns. 



The enamel investing the molar teeth is thick and strongly rugose, except on the 

 inferior first and second true molars, on which it is thinner and nearly smooth. 



Canines and incisors. — The canine teeth are proportionately about as well 

 developed as in the Wolves, hold the same relative position, and have nearly the 

 same form. Their long curved conical crown is invested with thick and strongly 

 rugose enamel. The acute linear ridges defining the inner from the outer surface, in 

 ordinary carnivora, appear to be obsolete in Hycenodon horridus. 



The upper incisor teeth of the two sides form a nearly transverse row, separated 

 from the contiguous canines by an interval sufficiently large to accommodate the 

 points of the canines below. They successively increase in size from first to last, and 

 curve outward and downward. They resemble those of the Weasels in form. The 

 intermediate ones have the crown convex in front, bevelled behind, and triangular at 

 the sides. The lateral incisor, twice the size of the others, has a more conical crown. 

 In the fossil they are all worn to the same level. 



The lower incisor teeth, as in the Weasels, appear crowded into the narrow space 

 of the contiguous canines, the second or intermediate one on each side being placed 

 internally to the interval of the two others, apparently as if it had been pushed out 

 of the row. They also hold the same relation of size to one another as in the 

 Weasels — increasing successivelj' from first to last. The crowns are worn and 

 mutilated in the fossil, but appear to have had nearly the same form as in the 

 Weasels. 



The measurements taken from the specimens of Hycenodon horridus will be given 

 hereafter in comparison with those of the smallest species of the genus from Dakota, 

 H. crucians. 



Prof Hayden's last collection from the Mauvaises Terres of Dakota has afforded 

 me an opportunity of examining another skull of Hycenodon horridus. The specimen 

 is nearly complete, though much fractured. A portion of the occiput, the zygomata, 

 and the back portions of the lower jaw, are the principal parts wanting. The jaws 

 are closely interlocked, and most of the inferior molars are concealed within the 

 included mass of matrix. 



The skull belonged to an adult past maturity, and the teeth are considerably worn. 

 It does not differ in any important character from those previously described, except 



