SCIUEOMORPHA. 847 



the last molar. The foramen mentale is situated near the superior plane of 

 the diastema, half way between the alveolar borders of the incisor and pre- 

 molar teeth. 



The grinding face of the molar series rises slightly anteriorly. The 

 sections of the middle two molars are subquadrate and are of equal size. 

 That of the fourth is smaller by one-third; having the same longitudinal 

 extent, but less width, thus forming a longitudinal oval. The section of the 

 premolar is round, and its diameter is half that of the long diameter of the 

 other molars. The premolar has two contiguous roots, and three conic 

 cusps, one anterior and two opposite and posterior. The cusps of the other 

 Tuolars are separated transversely by a deep notch, and longitudinally by a 

 iissure, which does not invade the surface of the cingulum. The latter 

 extends a short distance round the anterior base of the crowns of the second 

 and third molars. The crown of the fourth molar is worn or injured in my 

 specimens, so as not to be described. The incisor is strongly compressed, 

 -as in many Muridai, and the anterior face is slightly convex in section. 

 The enamel is smooth, and is abruptly recurved in narrow borders of about 

 *qual width on both the inner and outer sides. On the inner side the sur- 

 face makes a sharp right angle; on the outer side the angle is right, but is 

 obtusely rounded. The surface of attrition is elongate, and displays a 



small pulp cavity. 



Measurements. 



M. 



Length of incisive alveolus to end of molar series 0050 



lieugtli from incisive alveolus to base of ascending ramus 0044 



Xeugtli of diastema 0020 



JLengtli of third molar 0008 



"Width of third molar OOOd 



Long diameter of incisor OOOS 



.Short diameter of incisor 0004 



Depth of ramus at diastema 0018 



Depth of ramus at third molar 0021 



Thickness of ramus below the same 0014 



The Heliscomys vetus is the least mammal of the fauna of the White 

 Hiver epoch. The mandibular ramus is the size of that of the Mus mus- 

 'Culus, and its four teeth occupy the same length as the three of the latter 

 species. One of the specimens indicates an individual a little larger than 

 the one above measured. 



