﻿SONNINIA 
  AND 
  HAMMATOCERAS. 
  G57 
  

  

  and 
  the 
  deep 
  siphonal 
  lobe 
  and 
  the 
  obliquely 
  directed 
  inner 
  lobes 
  

   are 
  constant 
  features 
  not 
  found 
  in 
  anything 
  like 
  this 
  degree 
  in 
  

   Sonninia. 
  Fig. 
  17 
  shows 
  how 
  early 
  these 
  characteristics 
  are 
  pro- 
  

   duced 
  and 
  how 
  this 
  suture-line 
  differs 
  from 
  fig. 
  7. 
  These 
  characters 
  

   are 
  found 
  in 
  the 
  genera 
  Beroceras, 
  Stephanoceras, 
  Splicer 
  ocer 
  as, 
  &c, 
  

   showing 
  to 
  what 
  Hammatoceras 
  is 
  related 
  *. 
  

  

  Pigs. 
  13-15 
  show 
  the 
  sutures 
  of 
  Haucjia. 
  The 
  lobes 
  are 
  much 
  

   broader-stemmed, 
  and 
  do 
  not 
  exhibit 
  the 
  peculiar 
  cruciform 
  arrange- 
  

   ment 
  of 
  the 
  trifurcations 
  seen 
  in 
  the 
  lateral 
  lobe 
  of 
  Sonninia, 
  while 
  

   the 
  auxiliary 
  lobes 
  are 
  different. 
  

  

  If 
  the 
  inner 
  whorls 
  of 
  a 
  highly 
  developed 
  Sonninia, 
  whose 
  

   ancestry 
  we 
  do 
  know, 
  were 
  to 
  indicate 
  the 
  changes 
  which 
  have 
  taken 
  

   place, 
  it 
  would 
  be 
  a 
  strong 
  argument 
  in 
  favour 
  of 
  the 
  correctness 
  of 
  

   the 
  surmises 
  which 
  we 
  have 
  drawn 
  from 
  the 
  inner 
  whorls 
  of 
  Sonn. 
  

   acanthodes. 
  This 
  is 
  exactly 
  what 
  we 
  find 
  to 
  be 
  the 
  case. 
  The 
  inner 
  

   whorls 
  of 
  Sonn. 
  corrugata, 
  Sow. 
  (Amm. 
  patella, 
  Waagen), 
  are, 
  in 
  

   miniature, 
  Sonn. 
  propinquans, 
  tiayle 
  ; 
  the 
  inner 
  whorls 
  of 
  the 
  latter 
  

   are, 
  in 
  miniature, 
  Sonninia 
  subtrigonata, 
  n. 
  sp. 
  : 
  while 
  the 
  inner 
  

   whorls 
  of 
  this 
  are, 
  in 
  miniature, 
  Sonninia 
  acanthodes. 
  The 
  first 
  

   two 
  species 
  occur 
  in 
  the 
  Sauzei-zcme, 
  the 
  two 
  latter 
  in 
  the 
  next 
  

   zone 
  below, 
  the 
  Concavum-zone. 
  

  

  In 
  considering 
  the 
  development 
  of 
  the 
  smooth 
  Amal. 
  prcestabilis 
  

   from 
  the 
  spinous 
  Amal. 
  gloriosus, 
  and 
  the 
  smooth 
  Sonn. 
  corrugata 
  

   from 
  the 
  spinous 
  Sonn. 
  acanthodes, 
  we 
  may, 
  I 
  think, 
  assume 
  that 
  the 
  

   gradual 
  extinction 
  of 
  spines 
  and 
  ribs 
  is 
  not 
  wholly 
  due 
  to 
  advancing 
  

   decrepitude. 
  Some, 
  if 
  not 
  a 
  very 
  large, 
  allowance 
  must 
  be 
  made 
  for 
  

   the 
  increase 
  in 
  the 
  complexity 
  of 
  the 
  suture-line 
  ; 
  and 
  I 
  fancy 
  it 
  is 
  

   not 
  too 
  much 
  to 
  say 
  that 
  the 
  failure 
  of 
  external 
  ornaments 
  is 
  in 
  a 
  

   great 
  measure 
  due 
  to 
  this 
  increase 
  in 
  the 
  complexity 
  of 
  the 
  suture- 
  

   line 
  — 
  it 
  being 
  a 
  matter 
  of 
  compensation 
  ; 
  while 
  the 
  increase 
  in 
  the 
  

   suture-complications 
  is 
  necessitated 
  by 
  the 
  change 
  of 
  whorl-shape 
  

   — 
  from 
  evolution 
  to 
  involution, 
  and 
  from 
  arched 
  to 
  flattened 
  sides. 
  

  

  The 
  thin, 
  compressed, 
  highly-developed 
  Amaltheus 
  died 
  out 
  as 
  

   soon 
  almost 
  as 
  it 
  appeared 
  ; 
  the 
  e 
  vol 
  ate 
  /SWmma-ancestor 
  with 
  its 
  

   large 
  whorls 
  lived 
  to 
  produce 
  descendants 
  in 
  the 
  Concavum- 
  and 
  

   Sauzei-zones 
  ; 
  while 
  Pleuroceras 
  , 
  which 
  we 
  may 
  regard 
  as 
  somewhat 
  

   intermediate 
  between 
  the 
  two 
  in 
  point 
  of 
  development 
  — 
  though 
  more 
  

   like 
  Sonninia 
  — 
  also 
  produced 
  a 
  descendant 
  in 
  the 
  Concavum-zone. 
  

  

  This 
  was 
  Pleuroceras 
  subspinatum, 
  Buckm.t, 
  the 
  descendant 
  of 
  

   Pleur. 
  haivskerense. 
  It 
  has 
  changed 
  very 
  little. 
  Its 
  whorls 
  are 
  

   still 
  quadrangular 
  and 
  are 
  a 
  trifle 
  more 
  compressed, 
  but, 
  singularly 
  

   enough, 
  without 
  more 
  inclusion 
  ; 
  the 
  ribs 
  are 
  smaller 
  and 
  more 
  

   numerous, 
  the 
  spines 
  are 
  therefore 
  necessarily 
  smaller 
  ; 
  the 
  sul- 
  

   cations 
  of 
  the 
  ventral 
  area 
  are 
  still 
  prominent 
  ; 
  but 
  the 
  crenulations 
  

   of 
  the 
  carina 
  are 
  almost 
  entirely 
  obsolete 
  — 
  they 
  can 
  just 
  be 
  seen 
  with 
  

   a 
  glass 
  in 
  some 
  cases 
  ; 
  the 
  suture-line 
  (fig. 
  3) 
  agrees 
  in 
  the 
  general 
  

  

  * 
  Of 
  the 
  species 
  now 
  placed 
  in 
  the 
  genus 
  Erycites 
  some 
  have 
  been 
  referred 
  to 
  

   Ste%)hanoceras 
  and 
  some 
  to 
  Hammatoceras. 
  The 
  suture-line 
  (fig. 
  21) 
  shows 
  the 
  

   characters 
  of 
  Hammatoceras, 
  but 
  differs 
  in 
  the 
  smallness 
  of 
  the 
  siphonal 
  lobe, 
  the 
  

   functions 
  of 
  which 
  have 
  been 
  usurped 
  by 
  the 
  superior 
  lateral 
  lobe. 
  

  

  t 
  Proc. 
  Dorset 
  Club, 
  vol. 
  iv. 
  pi. 
  ii. 
  (1883). 
  

  

  