﻿PROM 
  THE 
  WEALDEN 
  FORMATION 
  IN 
  THE 
  ISLE 
  OP 
  WIGHT. 
  517 
  

  

  an 
  angle 
  of 
  about 
  35° 
  ; 
  and 
  it 
  crosses 
  the 
  long 
  axis 
  of 
  the 
  distal 
  end 
  

   at 
  an 
  angle 
  which 
  in 
  this 
  specimen 
  I 
  estimate 
  roughly 
  at 
  about 
  118°. 
  

   Below 
  the 
  knee 
  the 
  tibia 
  contracts 
  quickly 
  to 
  a 
  very 
  short 
  and 
  

   slender 
  shaft, 
  in 
  its 
  unexpanded 
  part 
  not 
  more 
  than 
  2| 
  to 
  3 
  inches 
  

   long, 
  and 
  with 
  a 
  minimum 
  girth 
  of 
  7 
  - 
  5 
  inches, 
  the 
  girth 
  of 
  tbe 
  prox- 
  

   imal 
  articular 
  end 
  measuring 
  21-5 
  inches. 
  The 
  cross 
  section 
  of 
  the 
  

   shaft 
  is 
  subtrigonal. 
  The 
  cortex 
  of 
  this 
  part 
  is 
  compact, 
  externally 
  

   smooth, 
  and 
  finely 
  grained. 
  No 
  large 
  medullary 
  canal 
  is 
  present, 
  as 
  

   in 
  Iguanodon 
  Mantelli, 
  but 
  a 
  moderately 
  fine 
  cancellated 
  tissue 
  

   nearly 
  fills 
  the 
  interior. 
  

  

  Downwards, 
  the 
  shaft 
  expanding 
  and 
  becoming 
  flattened, 
  merges 
  

   into 
  the 
  broad 
  triangular 
  ankle 
  end, 
  one 
  angle 
  or 
  malleolus 
  of 
  which 
  

   is 
  antero-internal, 
  the 
  other 
  postero-external, 
  while 
  the 
  surfaces 
  are 
  

   respectively 
  directed 
  outwards 
  and 
  forwards, 
  and 
  backwards 
  and 
  in- 
  

   wards. 
  The 
  antero- 
  external 
  surface 
  is 
  undivided 
  and 
  nearly 
  plane, 
  

   while 
  the 
  postero-internal 
  surface 
  is 
  broken 
  by 
  an 
  angulated 
  ridge 
  into 
  

   a 
  smaller 
  inner 
  and 
  a 
  wider 
  outer 
  part. 
  The 
  under 
  surface 
  of 
  this 
  end 
  

   is 
  pulley-shaped, 
  having 
  a 
  long, 
  narrow, 
  postero-external 
  and 
  a 
  shorter, 
  

   wider, 
  antero-internal 
  division. 
  Its 
  broad, 
  shallow, 
  trochlear 
  groove 
  

   ascends 
  upon 
  the 
  posterior 
  surface, 
  external 
  to 
  the 
  angulated 
  ridge 
  

   lately 
  mentioned 
  ; 
  and 
  opposite 
  to 
  it, 
  on 
  the 
  anterior 
  surface, 
  is 
  a 
  

   prominent 
  tubercle. 
  This 
  trochlear 
  part, 
  with 
  its 
  anterior 
  tubercle, 
  

   is 
  evidently 
  homologous 
  with 
  the 
  Ic/uanodon's 
  separate 
  astragalus. 
  

   The 
  condition 
  of 
  the 
  specimen 
  does 
  not 
  allow 
  me 
  to 
  say 
  whether 
  

   this 
  representative 
  of 
  the 
  astragalus 
  and 
  the 
  distal 
  end 
  of 
  the 
  

   tibia 
  had 
  become 
  confluent 
  during 
  life, 
  or 
  whether 
  they 
  have 
  simply 
  

   become 
  undistinguishably 
  blended 
  by 
  post-mortem 
  pressure 
  and 
  

   mineralization. 
  The 
  fact 
  that 
  the 
  inner 
  division 
  of 
  this 
  trochlea 
  stops 
  

   short 
  of 
  the 
  antero-internal 
  angle 
  or 
  inner 
  malleolus, 
  and 
  is 
  not 
  now 
  

   coextensive 
  with 
  the 
  entire 
  under 
  surface 
  of 
  this 
  part 
  of 
  the 
  tibia, 
  is 
  

   suggestive 
  of 
  a 
  post-mortem 
  displacement, 
  and, 
  if 
  so, 
  of 
  the 
  absence 
  

   of 
  a 
  genuine 
  bony 
  anchylosis. 
  

  

  The 
  proportions 
  of 
  this 
  tibia 
  are, 
  within 
  my 
  knowledge, 
  only 
  re- 
  

   peated 
  in 
  the 
  tibiae 
  of 
  Hylceosaurus 
  and 
  Polacanihus 
  Foxii. 
  The 
  

   materials 
  for 
  comparison 
  are 
  limited 
  to 
  a 
  single 
  example 
  of 
  each 
  ; 
  and 
  

   the 
  account 
  of 
  the 
  tibia 
  of 
  the 
  latter 
  in 
  the 
  Rev. 
  W. 
  Fox's 
  narrative 
  

   of 
  his 
  discovery 
  of 
  a 
  great 
  part 
  of 
  the 
  skeleton 
  of 
  this 
  remarkable 
  

   armour-clad 
  Saurian, 
  communicated 
  by 
  him 
  to 
  the 
  British 
  Associa- 
  

   tion 
  in 
  1866, 
  as 
  also 
  the 
  short 
  memorandum, 
  with 
  a 
  sketch 
  of 
  a 
  few 
  

   of 
  the 
  bones, 
  which 
  appeared 
  in 
  the 
  ' 
  Illustrated 
  London 
  News 
  ' 
  of 
  

   that 
  year, 
  are 
  rather 
  " 
  Preliminary 
  Notices," 
  which 
  scarcely 
  claim 
  

   to 
  afford 
  the 
  details 
  so 
  necessary 
  to 
  the 
  anatomist 
  who 
  cannot 
  study 
  

   the 
  actual 
  specimen. 
  

  

  Table 
  of 
  Measurements. 
  

  

  inches. 
  

  

  Length 
  of 
  tibia, 
  including 
  distal 
  epiphysis 
  or 
  astragalus 
  16 
  

  

  Long 
  diameter 
  of 
  proximal 
  end 
  7 
  

  

  Girth 
  of 
  ditto 
  21-5 
  

  

  Girth 
  of 
  shaft 
  7'5 
  

  

  Long 
  diameter 
  of 
  distal 
  end 
  8*25 
  

  

  