﻿some 
  Revialns 
  of 
  Rodents. 
  189 
  

  

  Dimensions 
  : 
  — 
  

  

  mm. 
  

  

  Alveolar 
  length 
  of 
  cheek-teeth 
  39 
  

  

  Breadth 
  of 
  incisor 
  dS 
  

  

  ^ 
  at 
  crown 
  11x8-5 
  

  

  Diastema 
  34 
  

  

  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.Jibera 
  mode- 
  

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

   dens 
  seems 
  thus 
  to 
  have 
  rapidly 
  acquired 
  a 
  dental 
  feature 
  

   which 
  the 
  recent 
  beaver, 
  had 
  it 
  been 
  left 
  alone, 
  might 
  have 
  

   developed 
  in 
  the 
  fullness 
  of 
  time. 
  

  

  2. 
  Trogontherium 
  cuvieri. 
  

  

  Among 
  the 
  specimens 
  from 
  the 
  Upper 
  Fresli 
  water 
  Bed 
  of 
  

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

   mentary 
  and 
  very 
  young 
  incisors 
  which, 
  despite 
  their 
  small 
  

   size, 
  apparently 
  belong 
  to 
  the 
  Trogontherium. 
  Two 
  of 
  them 
  

   (PI. 
  VIII. 
  figs. 
  I 
  & 
  5) 
  are 
  sharply 
  curved 
  and 
  are 
  evidently 
  

   left 
  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. 
  

   The 
  larger 
  specimen 
  has 
  been 
  in 
  use 
  long 
  enough 
  to 
  have 
  

   lost 
  by 
  wear 
  that 
  portion 
  of 
  the 
  crown 
  which 
  is 
  represented 
  

   by 
  the 
  smaller 
  tooth, 
  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 
  upper 
  incisor 
  just 
  described. 
  

   It 
  is 
  straighter 
  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 
  tinely 
  rugose 
  ; 
  there 
  is 
  a 
  narrow 
  

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

   on 
  the 
  inner 
  surface. 
  The 
  tip 
  (fig. 
  6a 
  & 
  h) 
  is 
  quite 
  unworn 
  

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

   lower 
  outer 
  one 
  — 
  separated 
  from 
  each 
  other 
  by 
  an 
  anterior 
  

   and 
  by 
  a 
  posterior 
  valley. 
  These 
  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 
  

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

  

  