﻿crick: 
  on 
  ammonites 
  NAVICULARIS, 
  MANTEl.L. 
  159 
  

  

  occupied 
  by 
  tlie 
  body-clianibcr 
  : 
  — 
  Diameter 
  of 
  shell, 
  (iGrani. 
  (1); 
  

   lieigbt 
  of 
  outer 
  wliorl, 
  29 
  inin. 
  (0'44) 
  ; 
  thickness 
  of 
  outer 
  whorl, 
  

   32 
  mm. 
  (0-48); 
  width 
  of 
  umbilicus, 
  18 
  mm. 
  (0"27). 
  The 
  ribs 
  are 
  as 
  

   a 
  rule 
  alternately 
  longer 
  and 
  sliorter, 
  but 
  there 
  is 
  some 
  irregularity 
  

   in 
  this 
  respect, 
  there 
  being 
  sometimes 
  two 
  smaller 
  ones 
  interpolated 
  

   between 
  a 
  pair 
  of 
  longer 
  ribs; 
  the 
  ribs 
  have 
  a 
  nearly 
  radial 
  direction, 
  

   tliey 
  are 
  straiglit 
  up 
  to 
  a 
  point 
  about 
  two-thirds 
  of 
  tlie 
  width 
  of 
  tlie 
  

   lateral 
  area 
  from 
  the 
  umbilical 
  margin, 
  where 
  they 
  bend 
  feebly 
  

   forward 
  as 
  far 
  as 
  the 
  margin 
  of 
  the 
  periphery, 
  and 
  then 
  pass 
  

   straight 
  across 
  the 
  latter. 
  In 
  passing 
  up 
  the 
  inner 
  area 
  of 
  the 
  

   whorl 
  each 
  longer 
  rib 
  gradually 
  increases 
  in 
  strength 
  up 
  to 
  the 
  

   umbilical 
  margin, 
  wheie 
  it 
  is 
  raised 
  into 
  a 
  distinct 
  elongated 
  

   compressed 
  tubercle 
  ; 
  each 
  of 
  these 
  ribs, 
  therefore, 
  lacks 
  the 
  distinct 
  

   Intuberculate 
  character 
  of 
  the 
  longer 
  ribs 
  on 
  the 
  originals 
  of 
  Sharpe's 
  

   hgs. 
  1 
  and 
  2. 
  Up 
  to 
  a 
  diameter 
  of 
  about 
  57 
  mm. 
  each 
  rib 
  is 
  slightly 
  

   angular 
  at 
  the 
  margin 
  of 
  the 
  periphery, 
  but 
  cannot 
  be 
  described 
  as 
  

   tuberculated 
  ; 
  beyond 
  this 
  diameter 
  the 
  angularity 
  gradually 
  dis- 
  

   appears. 
  At 
  a 
  point 
  nearly 
  in 
  the 
  middle 
  of 
  the 
  lateral 
  area, 
  but 
  

   a 
  little 
  nearer 
  the 
  periphery 
  than 
  tlie 
  umbilical 
  margin, 
  each 
  rib 
  is 
  

   also 
  slightly 
  angular 
  up 
  to 
  about 
  the 
  same 
  diameter, 
  the 
  angularity 
  

   being 
  more 
  distinct 
  and 
  elevated, 
  and 
  even 
  amounting 
  almost 
  to 
  

   a 
  tubercle, 
  on 
  the 
  younger 
  portion 
  of 
  the 
  outer 
  whorl. 
  The 
  inner 
  

   whorls 
  are 
  not 
  displayed. 
  

  

  The 
  specimen 
  is 
  from 
  the 
  Lower 
  Chalk 
  of 
  Chardstock, 
  Somerset, 
  

   and 
  was 
  formerly 
  in 
  the 
  collection 
  of 
  Professor 
  Jolm 
  Morris, 
  It 
  is 
  

   marked 
  in 
  ink 
  (ap))arently 
  in 
  the 
  handwriting 
  of 
  Daniel 
  Sharpe). 
  

   "Chardstock, 
  J. 
  ^loriis, 
  PI. 
  18, 
  Figs. 
  3rt 
  and 
  i," 
  and 
  tliere 
  can 
  be 
  

   no 
  doubt 
  whatever 
  about 
  its 
  being 
  the 
  figured 
  specimen. 
  

  

  The 
  original 
  of 
  Sharpe's 
  fig. 
  5ff 
  has 
  the 
  following 
  dimensions: 
  — 
  

   Diameter 
  of 
  shell, 
  40 
  mm. 
  (1); 
  height 
  of 
  outerwhorl, 
  17 
  mm. 
  (0'42) 
  ; 
  

   thickness 
  of 
  outer 
  whorl, 
  21mm. 
  (0'52); 
  aiul 
  width 
  of 
  umbilicus, 
  

   12 
  mm. 
  (0'30). 
  It 
  is 
  an 
  entirely 
  sejitate 
  specimen 
  with 
  thirty-one 
  

   ribs 
  in 
  the 
  outer 
  whorl. 
  As 
  a 
  rule 
  the 
  ribs 
  are 
  alternately 
  longer 
  and 
  

   shorter; 
  the 
  longer 
  arise 
  from 
  the 
  suture 
  of 
  the 
  shell 
  and 
  increasing 
  

   in 
  strength 
  as 
  they 
  cross 
  the 
  inner 
  area 
  (or 
  umbilical 
  wall) 
  of 
  the 
  

   whorl 
  are 
  raised 
  at 
  the 
  umbilical 
  margin 
  into 
  a 
  prominent 
  compressed 
  

   tubercle, 
  thence 
  eacli 
  rib 
  has 
  an 
  almost 
  j-adial 
  direction 
  as 
  far 
  as 
  the 
  

   middle 
  of 
  the 
  lateral 
  area, 
  where 
  each 
  rib 
  is 
  raised 
  into 
  a 
  transversely 
  

   compressed 
  tubercle, 
  the 
  lib 
  then 
  turning 
  slightly 
  forward 
  and 
  

   crossing 
  the 
  periphery 
  in 
  a 
  feeble 
  forwardly-couvex 
  curve. 
  Neiir 
  

   the 
  commencement 
  of 
  tlie 
  outer 
  whorl, 
  i.e. 
  where 
  the 
  shell 
  has 
  

   a 
  diameter 
  of 
  about 
  17 
  mm 
  , 
  the 
  row 
  of 
  tubercles 
  on 
  each 
  margin 
  

   of 
  the 
  periphery 
  and 
  the 
  median 
  row 
  of 
  tubercles 
  gradually 
  disappeai', 
  

   as 
  shown 
  in 
  Sliarpe's 
  fig. 
  bh, 
  but 
  belter 
  still 
  in 
  the 
  view 
  of 
  the 
  inner 
  

   whorls 
  represented 
  in 
  his 
  fig. 
  5c. 
  Beyond 
  this 
  point 
  there 
  is 
  on 
  

   either 
  margin 
  of 
  the 
  peripherv 
  only 
  a 
  very 
  feeble 
  angularity, 
  whicli 
  

   soon 
  disappears. 
  Unfortunately 
  the 
  inner 
  whorls 
  of 
  the 
  original 
  

   of 
  his 
  fig.- 
  3 
  are 
  not 
  shown, 
  but 
  so 
  far 
  as 
  can 
  be 
  seen 
  the 
  original 
  of 
  

   his 
  fig. 
  5 
  closely 
  resembles 
  that 
  specimen, 
  but 
  differs 
  in 
  having 
  

   slightly 
  more 
  inflated 
  whorls, 
  a 
  little 
  coarser 
  ornamentation, 
  with 
  

  

  VOL. 
  XIII.— 
  OCTOBER, 
  1919. 
  12 
  

  

  