﻿652 
  MR. 
  S. 
  S. 
  BTJCKMAN 
  ON 
  THE 
  DESCENT 
  OF 
  

  

  thodes 
  has 
  not 
  sprung 
  from 
  the 
  compressed 
  involute 
  Haugia 
  

   Ogerieni, 
  yet 
  that 
  the 
  same 
  parent 
  produced 
  both 
  forms. 
  

  

  At 
  first 
  sight 
  this 
  is 
  probable, 
  and 
  would 
  account 
  for 
  the 
  ventral 
  

   furrows 
  oi 
  Sonninia 
  ; 
  but 
  then 
  the 
  peculiar 
  inner 
  whorls 
  of 
  Sonninia, 
  

   with 
  their 
  flat, 
  broad, 
  ventral 
  area, 
  and 
  coronet 
  of 
  spines, 
  ought 
  to 
  

   bear 
  some 
  resemblance 
  to 
  the 
  inner 
  whorls 
  of 
  Haugia 
  Ogerieni, 
  but 
  

   they 
  do 
  not 
  bear 
  the 
  slightest. 
  The 
  inner 
  whorls 
  of 
  Haugia 
  

   Ogerieni 
  are 
  smooth 
  ; 
  while 
  the 
  knobs 
  — 
  not 
  spines 
  — 
  which 
  appear, 
  

   rise 
  on 
  the 
  inner 
  margin, 
  and 
  ribs 
  proceed 
  from 
  them 
  across 
  a 
  rather 
  

   flat 
  lateral 
  area. 
  

  

  The 
  supposition 
  that 
  Sonninia 
  propinquans 
  and 
  S. 
  Soiverbyi 
  are 
  

   descended 
  from 
  Hammatoceras 
  is 
  founded 
  on 
  the 
  peculiar 
  resem- 
  

   blance 
  between 
  Sonninia 
  Soiverbyi 
  and 
  Hammatoceras 
  subinsigne. 
  

   Looking 
  at 
  the 
  matter 
  from 
  a 
  chronological 
  point 
  of 
  view, 
  the 
  

   descent 
  of 
  Sonninia 
  from 
  Hammatoceras 
  is 
  very 
  suggestive. 
  The 
  

   latter 
  dies 
  out 
  in 
  the 
  Concavum-zone 
  — 
  that 
  is, 
  just 
  at 
  the 
  time 
  when 
  

   the 
  former 
  first 
  appears 
  ; 
  but 
  here, 
  again, 
  Sonninia 
  acanthodes 
  and 
  

   the 
  species 
  like 
  it 
  are 
  stumbling-blocks. 
  

  

  Practically 
  speaking, 
  I 
  have 
  not 
  found 
  any 
  sign 
  of 
  a 
  similar 
  

   sulcated 
  ventral 
  area 
  among 
  the 
  young 
  or 
  adults 
  of 
  Hammatoceras. 
  

   Another 
  small, 
  but 
  none 
  the 
  less 
  important, 
  point 
  is 
  that 
  the 
  ribs 
  of 
  

   Sonninia 
  have 
  a 
  prominent 
  forward 
  sweep 
  on 
  the 
  ventral 
  area 
  ; 
  but 
  

   the 
  ribs 
  of 
  Hammatoceras 
  meet 
  the 
  carina 
  almost 
  at 
  right 
  angles 
  — 
  

   in 
  fact 
  they 
  generally 
  appear 
  as 
  if 
  the 
  carina 
  had 
  cut 
  cleanly 
  through 
  

   them. 
  However, 
  the 
  chief 
  point 
  is 
  the 
  suture-line. 
  The 
  character- 
  

   istics 
  of 
  the 
  suture-line 
  of 
  Hammatoceras 
  (PI. 
  XXII. 
  figs. 
  16-20) 
  

   are 
  the 
  narrowness 
  and 
  depth 
  of 
  the 
  siphonal 
  saddle, 
  and 
  the 
  oblique 
  

   direction 
  of 
  the 
  inner 
  lobes. 
  If 
  Sonninia 
  were 
  descended 
  from 
  Ham- 
  

   matoceras 
  it 
  ought 
  to 
  show 
  some 
  of 
  these 
  characters, 
  or 
  some 
  change 
  

   from 
  them 
  which 
  could 
  be 
  explained 
  ; 
  but 
  it 
  shows 
  (figs. 
  6-10) 
  a 
  

   shallower 
  and 
  broader 
  siphonal 
  saddle, 
  and 
  its 
  inner 
  lobes 
  are 
  less 
  

   developed 
  and 
  more 
  nearly 
  in 
  a 
  straight 
  line 
  with 
  the 
  others. 
  We 
  

   might 
  reasonably 
  expect 
  to 
  find 
  a 
  suture-line 
  with 
  straighter 
  inner 
  

   lobes 
  in 
  Hammatocerata 
  which 
  had 
  acquired, 
  like 
  H. 
  amaltheiforme 
  

   (Yacek), 
  a 
  wider 
  lateral 
  area, 
  although, 
  as 
  it 
  happens, 
  we 
  do 
  not 
  j 
  

   but 
  Sonninia 
  acanthodes 
  is, 
  practically, 
  less 
  involute 
  than 
  Hamm. 
  

   insigne, 
  and 
  especially 
  than 
  H. 
  subinsigne, 
  and 
  has 
  a 
  narrower 
  

   lateral 
  area 
  in 
  which 
  to 
  accommodate 
  its 
  lobes. 
  "What 
  reason 
  is 
  there 
  

   for 
  the 
  suture-line 
  to 
  have 
  so 
  changed 
  from 
  Hamm. 
  subinsigne 
  or 
  

   Sieboldi 
  to 
  Sonninia 
  Sowerbyi, 
  while 
  the 
  outward 
  shape 
  has 
  changed 
  

   so 
  little 
  ? 
  

  

  If, 
  however, 
  we 
  turn 
  our 
  attention 
  to 
  the 
  genera 
  Pleuroceras 
  and 
  

   Amaltheus, 
  we 
  shall 
  find 
  ourselves 
  able 
  to 
  trace 
  most 
  of 
  the 
  dis- 
  

   tinctive 
  characters 
  of 
  Sonninia 
  in 
  species 
  of 
  those 
  genera 
  ; 
  and 
  we 
  

   shall 
  also 
  be 
  able 
  to 
  discover 
  a 
  definite 
  analogy 
  in 
  the 
  development 
  

   of 
  Sonninia 
  and 
  Amaltheus. 
  Further 
  we 
  shall 
  see 
  a 
  great 
  resem- 
  

   blance 
  between 
  the 
  most 
  developed 
  forms 
  of 
  Sonninia 
  and 
  Amaltlieus, 
  

   and 
  between 
  the 
  inner 
  whorls 
  of 
  the 
  less-developed 
  Sonninios 
  and 
  

   the 
  inner 
  whorls 
  of 
  Pleuroceras. 
  

  

  Amaltheus 
  and 
  Pleuroceras 
  are 
  undoubtedly 
  descended 
  from 
  one 
  

   common 
  ancestor. 
  "Where 
  that 
  ancestor 
  may 
  be 
  I 
  cannot 
  undertake 
  

  

  