126 Troxell — Rodents of Genus Ischyromys. 



corner forms the crown of this tooth, which in shape and 

 size resembles the lead of a pencil, an elongated cylinder. 



P 4 is distinctly molarif orm, but small and nearly round. 

 Unlike the molars, it has no internal groove and the cross 

 crests extend more than halfway across the crown. This 

 tooth now stands vertical in the maxillary, but had it 

 grown to its normal position, it would have turned out- 

 ward, following the curve of the large interior root, to 

 face more nearly like the molars. 



M 1 and M 2 are alike in general form. In each, the pos- 

 terior groove, after slight wear, becomes a pit near the 

 center of the posterior side. M 3 seems to be slightly 

 rotated backward and inward, giving the cross crests an 

 oblique direction; it is elongated fore and aft. The tri- 

 angular area about the posterior pit is small. 



The lower molars have the two simple outer cusps with 

 the two main, parallel cross crests leading from them. 

 The crowns are higher and the teeth are larger and wider 

 than in I. typus nanus just described. The last molar is 

 the widest ; the first is narrowest and the total length of 

 the three molars is 10 per cent greater than that of 

 I. nanus. In all the subspecies of /. typus there is a 

 notable absence of small secondary cusps. 



The first premolar was just appearing in the lower jaw 

 to take the place of the single deciduous tooth which had 

 been serving the young animal. Dp 4 is elongated but 

 narrow and has a low crown. The anterior cusps are not 

 paired transversely, for the inner one is placed far 

 anterior. ( See fig. 4 A.) 



The lower jaws are short and relatively deep ; the 

 ventral border forms a broad curve backward to a point 

 beneath M 3 ; here there begins a reverse curve and the 

 thin edge is folded inward to lend support to the pointed 

 angle. The coronoid extends as far above the condyle as 

 the angle does below; the two together give a vertical 

 measurement of about 26 mm. to the posterior part of the 

 ramus. On the upper side of the ridge leading to the 

 condyle opens the mandibular foramen ; this passage then 

 leads down and forward, following along the upper sur- 

 face of the incisor root within the ramus, and finally 

 emerges in the mental foramen just in front of P 4 . 

 Except for the greater height of the ascending ramus, the 

 extended angle, and the lack of a distinct muscle scar on 



