﻿660 
  ME. 
  S. 
  S. 
  BTJCK3IAX 
  OX 
  THE 
  DESCENT 
  OP 
  

  

  ceras 
  armatum 
  passed 
  into 
  Hamm. 
  insigne. 
  His 
  figures 
  of 
  Amm. 
  

   armatus 
  nodqfissus 
  (Amm. 
  Schwabischen 
  Jura, 
  pi. 
  xxvi. 
  figs. 
  8-13) 
  

   give 
  us 
  an 
  important 
  clue 
  — 
  the 
  spines 
  or 
  knobs 
  have 
  crept 
  very 
  

   near 
  to 
  the 
  inner 
  margin, 
  and 
  the 
  form 
  is 
  becoming 
  involute. 
  

  

  If 
  we 
  imagine 
  this 
  form 
  to 
  have 
  proceeded 
  one 
  step 
  further 
  in 
  

   development 
  in 
  the 
  ordinary 
  way, 
  it 
  would 
  have 
  become 
  more 
  

   involute, 
  and 
  the 
  spines 
  or 
  knobs 
  would 
  have 
  disappeared. 
  The 
  

   young 
  oiHanim. 
  insigne 
  are 
  thick, 
  involute, 
  and 
  have 
  no 
  knobs 
  on 
  

   the 
  inner 
  margin. 
  ]N 
  T 
  ow 
  comes 
  the 
  reversion. 
  Instead 
  of 
  continuing 
  

   to 
  become 
  more 
  involute, 
  the 
  young 
  Hamm. 
  insigne 
  grows 
  a 
  wider 
  

   umbilicus, 
  and 
  at 
  the 
  same 
  time 
  reassumes 
  the 
  spines 
  or 
  knobs 
  

   upon 
  the 
  inner 
  margin. 
  But 
  the 
  greater 
  umbilical 
  width 
  is 
  not 
  

   accompanied 
  by 
  any 
  increase 
  in 
  the 
  thickness 
  of 
  the 
  whorls, 
  so 
  

   Hamm. 
  insigne 
  increases 
  but 
  little 
  in 
  thickness 
  after 
  the 
  period 
  of 
  

   the 
  change. 
  Both 
  young 
  and 
  old 
  differ 
  from 
  Amm. 
  nodqfissus 
  by 
  the 
  

   possession 
  of 
  a 
  small 
  solid 
  carina 
  ; 
  my 
  specimens 
  do 
  not 
  allow 
  me 
  

   to 
  say 
  when 
  this 
  commenced. 
  

  

  We 
  thus 
  have 
  three 
  stages 
  in 
  the 
  development 
  of 
  Hamm. 
  insigne, 
  

   the 
  armatus-stage, 
  the 
  nodofissus-stage, 
  and 
  the 
  insignis-stage. 
  The 
  

   next 
  development 
  of 
  Hammatoceras 
  may 
  be 
  called 
  the 
  subinsignis- 
  

   stage 
  ; 
  and 
  it 
  is 
  noticeable 
  that, 
  in 
  the 
  growth 
  of 
  the 
  subinsignis-i 
  'orms, 
  

   the 
  nGclofissus-st&ge 
  — 
  the 
  intermediate 
  or 
  involute 
  stage 
  — 
  is 
  omitted 
  

   altogether, 
  so 
  that 
  we 
  should 
  find 
  no 
  clue 
  to 
  it 
  in 
  the 
  inner 
  whorls*. 
  

   The 
  subinsignis 
  -stage 
  shows 
  a 
  still 
  further 
  reversion 
  — 
  the 
  knobs 
  grow- 
  

   ing 
  into 
  actual 
  spines, 
  and 
  coming 
  more 
  into 
  the 
  middle 
  of 
  the 
  lateral 
  

   area 
  — 
  seen 
  to 
  most 
  advantage 
  in 
  Hamm. 
  dolium, 
  n. 
  sp. 
  Prom 
  the 
  

   small 
  solid 
  carina 
  of 
  insignis 
  this 
  stage 
  develops 
  the 
  large 
  trenchant 
  

   7wlloiu 
  -caima, 
  : 
  while 
  it 
  foreshadows 
  the 
  next 
  or 
  amaltheiforme-st&ge 
  

   in 
  its 
  broader, 
  more 
  compressed 
  whorls 
  losing 
  their 
  ornaments, 
  and 
  

   this 
  trenchant 
  carina, 
  when 
  adult. 
  

  

  This 
  last 
  stage 
  is 
  seen 
  to 
  most 
  perfection 
  in 
  Hamm. 
  amplectens, 
  

   n. 
  sp., 
  in 
  which 
  involution 
  has 
  attained 
  its 
  utmost 
  limit 
  — 
  short 
  of 
  a 
  

   closed 
  umbilicus 
  ; 
  while 
  the 
  whorl 
  is 
  discoidal, 
  and 
  almost 
  without 
  

   any 
  ornaments 
  when 
  adult. 
  

  

  Another 
  branch 
  of 
  Hammatoceras 
  developed 
  into 
  Hamm. 
  plan- 
  

   insigne, 
  Yacek 
  ; 
  this 
  into 
  Hamm. 
  tenuinsigne, 
  Yacek 
  ; 
  and 
  this 
  into 
  

   Hamm. 
  climacomphalum 
  (Vacek), 
  with 
  the 
  same 
  result 
  — 
  the 
  pro- 
  

   duction 
  of 
  an 
  involute, 
  discoidal, 
  almost 
  unornamented 
  form. 
  

  

  In 
  Hamm. 
  amplectens 
  and 
  Hamm. 
  climacompludum, 
  which 
  are 
  

   the 
  most 
  changed 
  forms 
  of 
  the 
  genus, 
  we 
  see 
  the 
  same 
  discoidal 
  

   shape, 
  with 
  small 
  umbilicus, 
  which 
  appertains 
  to 
  the 
  most 
  developedt 
  

   forms 
  of 
  so 
  many 
  genera, 
  such 
  as 
  Amaltheus 
  i^ra^stabilis, 
  Sonninia 
  

   corrvyata, 
  Lioceras, 
  Hyperlioceras. 
  The 
  former 
  even 
  bears 
  great 
  

   resemblance 
  to 
  Oppelia 
  subradiata 
  ; 
  while 
  the 
  latter 
  has 
  an 
  extra- 
  

  

  * 
  Traces 
  of 
  an 
  intermediate 
  stage 
  are 
  often 
  omitted. 
  Thus 
  Lioceras 
  shows 
  

   no 
  traces 
  of 
  the 
  sulcate 
  ventral 
  area 
  of 
  its 
  Arietan 
  -ancestor. 
  

  

  t 
  This 
  term 
  is 
  used 
  somewhat 
  arbitrarily 
  to 
  express 
  the 
  species 
  which 
  have 
  

   gone 
  through 
  the 
  greatest 
  number 
  of 
  changes 
  to 
  arrive 
  at 
  their 
  shape, 
  without 
  

   taking 
  into 
  consideration 
  whether 
  the 
  later 
  changes 
  are 
  a 
  decadence 
  or 
  other- 
  

   wise. 
  

  

  