23G ON THE EXTINCT MAMMALIA OF 



jaws form unbroken arches, of which that of the lower jaw is the smaller. The 

 arches are about two inches in length. 



The hiatus separating the upper molars from the canines forms a concave notch, 

 with an acute edge, and varies from half an inch to an inch in extent. That of the 

 lower jaw is of the same character and extent. 



The superior molars of Hyracodon nehrascensis so closely resemble those of Rhino- 

 ceros occidentalis that a description would ajjply nearly equally well for both species. 

 The last tooth of the series in the foi'mer, however, differs strikingly in exhibiting a 

 decided tendency to the formation of a posterior valley to the crown, or to the dis- 

 tinction of a pair of inner lobes from the outer portion of the crown. (Compare 

 figure 13, plate xiv, and figure 3, plate xv, with figure 1, plate xii, of the Ancient 

 Fauna of Nebraska.) 



The inferior molars also agree in form with those of B. occidentalis, but have the 

 basal ridge relatively better developed. In a specimen of the lower jaw of an adult 

 individual, a portion of a small first premolar is preserved, sufficient to indicate that 

 seven lower molars belong to the series. 



In all the specimens under examination the crowns of the incisors and canines 

 have been lost except in the case of a single incisor, — the last of the upper series. 

 The crown of this tooth is simply conical and pointed, and is four lines long. A 

 small remaining fragment of the contiguous canine appears to indicate that this was 

 also conical, and I suspect that all the incisors and canines of both jaws had simple 

 conical crowns. 



Judging from the fangs preserved in an upper jaw, the incisors appear slightly to 

 have diminished in size from first to last, while the canine was a little larger than 

 the contiguous incisor, though not more so than the first one. 



In two fragments of lower jaws, in which the fangs of the incisors and canines are 

 preserved, the former teeth appear to have slightly and successively increased in size 

 from the first of the series, but the canine more abruptly enlarged. 



The temporary molars of the upper jaw of St/racodon differ from their permanent 

 successors in having the internal lobes of the crown more equally developed, and in 

 being separated from each other to the base internally. The last inferior temporary 

 molar exhibits a disposition to form a third lobe situated in advance of the usual pair 

 of lobes. 



Measurements derived from a skull of Hyracodon nehrascoisis are as follow : 



Inches. 



Length of skull from occipital condyle to incisive alveoli, . . . lOJ 



Breadth of inion at mastoid processes, . . .• . .3 



Breadth at zygomata, . . . . . . . 5J 



Distance between tips of post-glenoid processes, . . . .3 



