some Remains of Rodents. 1S9 



Dimensions : — 



mm. 



Alveolar length of cheek-teeth 39 



Breadth of incisor 9-3 



^ at crown 1 1 X 80 



Diastema 3i 



C. ph'cidens appears to be a specialized offshoot which 

 arose and became extinct in the Upper Pliocene period. 

 Forsyth Major tells us that in aged teeth of C.jiherix mode- 

 rate plication of the enamel makes its appearance. G. plici- 

 dens seems thus to have rapidly acquired a dental feature 

 w hich the recent beaver, had it been left alone, might have 

 developed in the fullness of time. 



2. Trogoniher'ium cuvieri. 



Among the specimens from the Upper Fresli water Bed of 

 West Runton in Mr. Savin's private collection are three frag- 

 mentary and very young incisors which, despite their small 

 size^ apparently belong to the Trogontherium. Two of them 

 (PI. Vlll. ligs. 4 & 5) are sharply curved and are evidently 

 Ifft upper incisors. Their sectional diameters increase rapidly 

 posteriorly, and they are characterized by their convex faces 

 of rugose enamel. The outer surface in each is traversed by 

 a narrow groove along the junction of the enamel with the 

 dentine, and there are fainter indications of a similar feature 

 on the inner surface. The tip of the smaller and younger 

 specimen presents a round, rather uneven, wearing surface. 

 Tiie larger specimen has been in use long enough to have 

 lost by wear that portion of the crown which is represented 

 by the smaller tootii, and, although the end is now somewhat 

 mutilated, to have acquired a normal disc of wear. 



The third specimen is very small, its broken base not being 

 wider than the tip of the smaller u{iper incisor just described. 

 It is straigliter than either of the others, and may be deter- 

 mined as the left lower incisor (Pi. VIII. fig. 6). The enamel 

 of the convex anterior face is finely rugose ; there is a narrow 

 groove on the outer and a still narrower and less distinct one 

 on the inner surface. The tip (fig, Qa &,h) is quite unworn 

 and consists of two little tubercles — a higher inner and a 

 luwer outer one — separated from each other by an anterior 

 and by a posterior valley. Tliese valleys extend downwards 

 upon the front and hinder surfaces of the tooth for a distance 

 of about 1 mm., and then die out. The posterior valley 

 (tig. 6 a) is a deeply re-entrant V-shaped notch, cutting the 



