DAKOTA AND NEBRASKA. 343 



approaching in size the Brown Rat, Mus decumanus. The species has been distin- 

 guished by the name at the head of the chapter. 



The jaw fragment, consisting of the intermediate portion, agrees in form and consti- 

 tution with the corresponding portion in the Rat. The inserted portion of the incisor, 

 retained in the specimen, projects as far back and in the same manner as in the Rat. 



The side of the jaw was provided with three molar teeth as in the latter animal, 

 but the intermediate one alone is preserved in the specimen, together with the alveoli 

 and fangs of the others. 



The entire molar has an oblong-square crown, inserted into the jaw by fangs as in 

 the Rat. The triturating surface, represented in figure 13, magnified four diameters, 

 exhibits four principal lobes entering into the constitution of the ci'own. The summits 

 of the lobes are worn so as to exhibit a continuous tract of dentine bordered by en- 

 amel, but unprovided with intervening cementum. The worn surfaces of the ovoidal 

 lobes alternate with each other internally and externally. Those of the anterior pair 

 of lobes conjoin by a narrow tract in front, extending into a point inwardly and out- 

 wardly. Those of the internal lobes are separated by a pointed tract extending into 

 the valley separating the latter. Those of the outer lobes are separated by the exter- 

 nal transverse valley. The worn surface of the postero-external lobe extends in a 

 pointed tract back of the postero-internal lobe. 



The measurements of the specimen are as follow : 



Lines. 



Depth of jaw below second molar, . . . . . .3 



Space occupied by three molars, . . , . . . 3i 



Antero-posterior diameter of second molar, . . . . .1 



EYSTRICID^. 



The family of the Porcupines is represented by a species of Eystrix, in the pliocene 

 fauna of the Niobrara River. 



HYSTRIX. 



Hystriz venustus. 



Two isolated molar teeth of a rodent animal, differing from any of the preceding, 

 were discovered by Dr. Hayden in company with the specimen of Castor tortus. 

 They apparently indicate a species of Porcupine, but in structure are unlike those of 

 the recent American Porcupine, Ereiliizon dorsatus, and exhibit a more evident rela- 

 tionship with those of the crested Porcupine, Hijstrix cristata, of Europe. 



One of the teeth, represented in figure 23, plate XXVI, magnified one and a half 

 diameters, corresponds with the first upper molar of the right side in the latter 



