NATURAL SCIENCES OF PHILADELPHIA. 65 



The space occupied by the mohir series was about 6} lines. The length of 

 the crown of the third premolar is one and one-lifth lines ; the breadth 1 line. 

 Length of crown of second premolar five-sixths of a line, breadth one and one- 

 fifth lines. Breadth of first true molars \\ lines. 



Unlike the corresponding part of any other insectivorous animal known to 

 me, I have referred the jaw fragment to a new genus, and have dedicated the 

 species to its discoverer, with the name of Omomys Carleri. 



2. NaNOHYUS P0UCINU8. 



In the expedition of Dr. F. V. Hayden, in the summer of 1866, to the Mau- 

 vaises Terres of White River, Dakota, among tlie fossil vertebrate remains 

 previously noticed or described, he discovered a fragment of the left ramus of 

 the lower jaw of a small mammal, supposed to be nearly allied, if not belong- 

 ing to the suiline family. 



The teeth in the fragment consist of the last temporary molar ; the suc- 

 ceeding two permanent molars in functional position, and the anterior por- 

 tion of the third molar partially protruded. The interior of the jaw beneath 

 the temporary molar is occupied by the crown of the last premolar, which, 

 judging from the appearance of the exposed outer part, has the same form and 

 size as the molars behind. 



The temporary molar is inserted by a pair of widely separated fangs, and 

 its crown presents the usual greater breadth than in the succeeding pair of those 

 of the teeth behind, as in pachyderms generally. The crown is trilobate ex- 

 ternally and internally, and this condition probably corresponds with three 

 constituent pairs of lobes, the distinction of which is for the most part oblite- 

 rated by wearing. The median division of the crown is largest, and that in 

 advance is the smallest. The abraded summit of the former presents an 

 irregularly transverse quadrate surface of exposed dentine continuous with a 

 smaller subreniform tract upon the anterior division. The posterior division 

 of the crown still exhibits the distinction of a transverse pair of lobes, of 

 which the outer one is much the larger. This presents on its abraded summit 

 a crescentoid surface of exposed dentine, and the inner one a minute circular 

 islet of the same substance, and both are considerably below the level of the 

 worn surfaces of the divisions of the crown in front. 



The two permanent true molars preserved in the specimen are alike in 

 form and size, and the anterior portion of the last molar agrees in character 

 with the corresponding portion of the teeth in advance. 



The crown of the first and second true molars is composed of two transverse 

 pairs of conical lobes of which the anterior are about a third higher than the 

 posterior, and are separated from them by a deep transverse valley. The 

 inner and outer lobes are separated by a valley almost half the depth of the 

 former, closed at the fore and back part of the crown by a small tubercle. The 

 front tubercle is most conspicuous, and receives a feeble offset or ridge from 

 the antero-external lobe. A similar offset from the postero-external lobe ends 

 in the middle of the transverse valley of the crown. A basal ridge exists no- 

 where except at the fore part of the crown, where it is most conspicuous exter- 

 nally and is there associated with the tubercle closing the front of the fore 

 and aft valley of the tooth. 



A minute circular islet of exposed dentine occupies the summits of the 

 anterior lobes of the crown of the first true molar. 



The breadth of the crown of the last temporary molar is 2\ lines. The 

 breadth of the crown of the second unworn permanent true molar is If lines, 

 and its height at the anterior division is the same. 



The depth of the jaw fragment below the first permanent true molar is one 

 fourth of an inch. The base is moderately convex fore and aft. 



Though I have found it difficult to ascertain, by comparison with figures, how 

 far the fossil described difiFers from the corresponding portion of other known 

 animals, it nevertheless appears to me to do so sufiBciently to refer it to a dis- 

 tinct genus and species, for which I have proposed the name of Nanohyus por- 

 cinus. 



1869.] 5 



