﻿S0NN1NIA 
  AND 
  HAMMATOCEEAS. 
  653 
  

  

  to 
  say 
  ; 
  but 
  the 
  form 
  of 
  the 
  ancestor 
  of 
  these 
  two 
  genera 
  may 
  be 
  

   gathered 
  if 
  we 
  imagine 
  a 
  very 
  considerable 
  enlargement 
  of 
  the 
  inner 
  

   whorls 
  of 
  Amaliheus 
  (fig. 
  25 
  is 
  enlarged 
  only 
  about 
  4 
  times). 
  

  

  This 
  specimen 
  may 
  be 
  described 
  as 
  possessed 
  of 
  a 
  simple 
  coronet 
  of 
  

   spines, 
  and, 
  practically, 
  an 
  uncarinate 
  ventral 
  area. 
  Both 
  Pleuro- 
  

   ceras 
  and 
  Amaltlieus 
  agree 
  in 
  having 
  similar 
  inner 
  whorls 
  which 
  

   develop 
  as 
  follows 
  : 
  — 
  

  

  Pleur. 
  sjpinatum, 
  when 
  broken 
  up, 
  shows 
  the 
  commencement 
  of 
  

   the 
  keel 
  at 
  2 
  lines, 
  with 
  a 
  broad 
  ventral 
  area 
  and 
  a 
  coronet 
  of 
  spines 
  

   up 
  to 
  4| 
  lines 
  ; 
  after 
  that 
  a 
  flattened 
  side 
  and 
  indications 
  of 
  a 
  

   second 
  row 
  of 
  spines 
  ; 
  sulcations 
  on 
  each 
  side 
  of 
  a 
  crenulated 
  carina 
  

   not 
  before 
  11 
  lines. 
  

  

  Pleuroceras 
  licavskerense 
  shows 
  single 
  spines 
  up 
  to 
  a 
  diameter 
  of 
  

   6 
  lines 
  ; 
  after 
  that 
  a 
  second 
  row 
  of 
  spines 
  accompanied 
  by 
  a 
  more 
  

   flattened 
  side. 
  

  

  Pleur. 
  pseudocostatum 
  shows, 
  when 
  broken 
  up, 
  a 
  ventral 
  area 
  

   without 
  carina, 
  a 
  lateral 
  area 
  with 
  folds 
  or 
  small 
  knobs, 
  and 
  an 
  

   aperture 
  somewhat 
  broad 
  ventrally 
  at 
  2 
  j 
  lines 
  ; 
  the 
  folds 
  or 
  knobs 
  

   developing 
  into 
  ribs 
  with 
  a 
  spine 
  towards 
  the 
  outer 
  end, 
  the 
  faintest 
  

   suspicion 
  of 
  carina, 
  the 
  sides 
  more 
  flattened 
  at 
  5 
  lines 
  ; 
  a 
  noticeable 
  

   carina, 
  prominent 
  ribs 
  with 
  a 
  forward 
  ventral 
  sweep 
  at 
  8 
  lines 
  ; 
  a 
  

   crenulated 
  carina 
  and 
  sulcated 
  ventral 
  area 
  not 
  before 
  14 
  lines. 
  

  

  Amaltheus 
  margaritatus 
  exhibits 
  an 
  uncarinate 
  ventral 
  area 
  and 
  a 
  

   coronet 
  of 
  single 
  spines 
  at 
  2 
  lines 
  diameter; 
  while 
  before 
  it 
  reaches 
  

   5 
  lines 
  the 
  sides 
  are 
  much 
  compressed. 
  

  

  We 
  see 
  from 
  the 
  above 
  that 
  a 
  form 
  with 
  uncarinate 
  ventral 
  area 
  

   and 
  a 
  coronet 
  of 
  spines 
  is 
  common 
  to 
  the 
  early 
  stages 
  of 
  Pleuroceras 
  

   and 
  Amaltheus. 
  We 
  can 
  trace 
  the 
  development 
  of 
  the 
  various 
  

   branches 
  therefrom 
  as 
  follow 
  : 
  — 
  The 
  AmaltJieus-br 
  c 
  <mch, 
  which 
  adds 
  

   first 
  a 
  crenulated 
  carina 
  and 
  a 
  slightly 
  flattened 
  whorl, 
  but 
  retains 
  

   the 
  spines 
  — 
  such 
  is 
  Amaltheus 
  gloriosus, 
  Hyatt. 
  From 
  this 
  species, 
  

   through 
  Amal. 
  turgidus 
  and^imaZ. 
  margaritatus 
  to 
  Amal. 
  prcestabilis 
  

   on 
  one 
  side, 
  and 
  to 
  Amal. 
  Engelhardti 
  on 
  the 
  other, 
  is 
  an 
  exact 
  course 
  

   of 
  development 
  — 
  the 
  descendants 
  ever 
  inheriting 
  their 
  parents' 
  cha- 
  

   racteristics 
  at 
  an 
  earlier 
  age 
  *. 
  The 
  tendency 
  of 
  this 
  development 
  

   is 
  to 
  flatten 
  and 
  broaden 
  the 
  whorl, 
  to 
  increase 
  the 
  involution, 
  and 
  

   to 
  supersede 
  the 
  spines. 
  

  

  The 
  Pleuroceras-\)r&nch. 
  must 
  be 
  divided 
  into 
  two 
  : 
  — 
  1st. 
  The 
  

   Pleur. 
  -ptseudocostatum-giouj), 
  in 
  which 
  the 
  side 
  becomes 
  flattened, 
  

   and 
  produces 
  a 
  rib 
  inside 
  the 
  spine, 
  while 
  the 
  ventral 
  area 
  remains 
  

   uncarinate 
  for 
  a 
  time, 
  and 
  in 
  which 
  the 
  ventral 
  sulcations, 
  after 
  the 
  

   advent 
  of 
  the 
  carina, 
  are 
  never 
  so 
  prominent 
  ; 
  2nd. 
  The 
  P. 
  hawsker- 
  

  

  * 
  When 
  I 
  wrote 
  (Monogr. 
  Inf. 
  Ool. 
  Amm., 
  Pal. 
  Soc. 
  1888) 
  my 
  remarks 
  con- 
  

   cerning 
  the 
  changes 
  observable 
  during 
  the 
  evolution 
  of 
  the 
  Hildoceratidse, 
  and 
  

   showed 
  how 
  these 
  followed, 
  as 
  it 
  were, 
  certain 
  definite 
  rules, 
  I 
  was 
  not 
  acquainted 
  

   with 
  anything 
  Prof. 
  Hyatt 
  had 
  written 
  upon 
  the 
  evolution 
  of 
  other 
  families 
  ; 
  

   and 
  this 
  accounts 
  for 
  my 
  not 
  having 
  made 
  any 
  mention 
  of 
  his 
  work. 
  Although 
  

   regretting 
  that 
  I 
  was 
  not 
  previously 
  acquainted 
  with 
  such 
  valuable 
  work, 
  yet 
  I 
  

   cannot 
  but 
  be 
  agreeably 
  pleased 
  to 
  find 
  that 
  my 
  observations 
  upon 
  the 
  mode 
  of 
  

   evolution 
  of 
  the 
  Hildoceratidre 
  agree 
  so 
  exactly 
  with 
  what 
  he 
  has 
  recorded 
  con- 
  

   cerning 
  other 
  families. 
  

  

  