﻿190 
  Mr. 
  M. 
  A. 
  C. 
  Hinton 
  on 
  

  

  crown 
  to 
  its 
  centre 
  ; 
  the 
  less 
  deeply 
  re-entrant 
  anterior 
  valley 
  

   (fig. 
  6h) 
  has 
  its 
  floor 
  raised 
  as 
  a 
  little 
  rib. 
  Each 
  valley 
  

   contains 
  a 
  little 
  cement. 
  The 
  hinder 
  sides 
  of 
  the 
  tubercles 
  

   and 
  the 
  posterior 
  valley 
  are 
  invested 
  with 
  a 
  thin 
  coat 
  of 
  

   enamel, 
  but 
  whether 
  the 
  latter 
  extends 
  over 
  the 
  apices 
  of 
  the 
  

   tubercles 
  or 
  not 
  is 
  uncertain. 
  The 
  grooves 
  of 
  the 
  out^r 
  and 
  

   inner 
  surfaces 
  — 
  persistent 
  features 
  in 
  adult 
  incisors 
  of 
  Trogon- 
  

   therium 
  — 
  are 
  seen 
  to 
  start 
  from 
  the 
  summit. 
  

  

  In 
  discussing 
  the 
  grooved 
  upper 
  incisors 
  of 
  Lagomorpha 
  

   Forsyth 
  Major 
  * 
  has 
  stated 
  that: 
  — 
  *' 
  Tlie 
  incisors 
  provided 
  

   with 
  enamel-folds 
  point 
  back 
  towards 
  cuspidate 
  incisors, 
  for 
  

   the 
  enamel-folds 
  of 
  lophodont 
  and 
  laminated 
  teeth 
  are 
  obviously 
  

   the 
  derivatives 
  and 
  homologues 
  of 
  the 
  'valleys' 
  separating 
  

   the 
  cusps 
  or 
  tubercles.^' 
  In 
  this 
  connection 
  it 
  is 
  of 
  very 
  

   great 
  interest 
  to 
  find 
  traces 
  of 
  the 
  primitive 
  complexity 
  in 
  

   the 
  unworn 
  incisor 
  of 
  Trogontherium. 
  It 
  may 
  be 
  that 
  in 
  

   some 
  cases, 
  where 
  one 
  or 
  more 
  grooves 
  channel 
  the 
  anterior 
  

   faces 
  of 
  the 
  incisors, 
  their 
  persistence 
  is 
  due 
  to 
  the 
  fact 
  that 
  

   such 
  grooves 
  are 
  of 
  use 
  to 
  the 
  animal 
  retaining 
  them, 
  as 
  

   Tullbergt 
  suggests 
  ; 
  but, 
  in 
  my 
  view, 
  it 
  is 
  wrong 
  to 
  regard 
  

   such 
  a 
  groove 
  as 
  a 
  new 
  feature 
  produced 
  in 
  any 
  given 
  case, 
  

   'because 
  it 
  is 
  there 
  useful. 
  It 
  is 
  a 
  fact 
  that 
  frequently 
  in 
  

   Lagomorpha 
  and 
  in 
  other 
  rodents 
  more 
  or 
  less 
  distinct 
  traces 
  

   of 
  a 
  posterior 
  incisor 
  valley 
  can 
  be 
  found 
  in 
  the 
  form 
  of 
  a 
  

   shallow 
  sulcus, 
  which, 
  devoid 
  of 
  enamel, 
  can 
  hardly 
  be 
  of 
  any 
  

   functional 
  importance. 
  Further, 
  in 
  many 
  voles 
  vestiges 
  of 
  

   vanished 
  valleys 
  can 
  be 
  found 
  as 
  narrow, 
  shallow, 
  well- 
  

   defined, 
  and 
  persistent 
  grooves 
  upon 
  the 
  sides 
  of 
  their 
  

   molars 
  ; 
  in 
  these 
  cases 
  it 
  is 
  only 
  by 
  a 
  study 
  of 
  unworn 
  or 
  

   little-worn 
  teeth 
  that 
  the 
  true 
  significance 
  of 
  such, 
  at 
  first 
  sight, 
  

   trivial 
  features 
  becomes 
  apparent. 
  

  

  One 
  of 
  the 
  most 
  interesting 
  of 
  Mr. 
  Savin's 
  recent 
  acquisi- 
  

   tions 
  from 
  the 
  West-Runton 
  deposit 
  is 
  a 
  foot-bone, 
  which 
  I 
  

   have 
  determined 
  as 
  the 
  right 
  navicular 
  of 
  Trogontherium 
  — 
  a 
  

   part 
  hitherto 
  unknown. 
  In 
  PI. 
  VIII. 
  fig. 
  7 
  four 
  views 
  of 
  

   the 
  fossil 
  are 
  given, 
  with 
  corresponding 
  views 
  (fig. 
  8) 
  of 
  

   the 
  navicular 
  of 
  Castor 
  fiber. 
  Generally 
  the 
  fossil 
  closely 
  

   agrees 
  with 
  the 
  recent 
  bone 
  in 
  form 
  and 
  in 
  the 
  number 
  and 
  

   arrangement 
  of 
  the 
  facettes 
  ; 
  it 
  differs 
  principally 
  in 
  having 
  

   the 
  articular 
  surfaces 
  much 
  larger, 
  the 
  posterior 
  spur, 
  on 
  the 
  

   other 
  hand, 
  much 
  smaller 
  — 
  so 
  that, 
  as 
  a 
  result, 
  its 
  absolute 
  

   size 
  is 
  but 
  little 
  greater. 
  Anteriorly 
  it 
  is 
  wider, 
  posteriorly 
  

  

  * 
  Forsytli 
  Major, 
  Trans. 
  Linn. 
  Soc, 
  Zool. 
  ser. 
  2, 
  vol. 
  vii. 
  p. 
  470 
  (1899), 
  

   t 
  TuUberg, 
  ' 
  Ueber 
  das 
  System 
  der 
  Nagethiere,' 
  p. 
  363. 
  

  

  