﻿654 
  ME. 
  S. 
  S. 
  BTJCKMAtf 
  ON 
  THE 
  DESCENT 
  OP 
  

  

  ense- 
  and 
  P. 
  spinatum- 
  group, 
  in 
  which 
  the 
  carina 
  commences 
  while 
  

   the 
  spines 
  remain 
  single, 
  and 
  in 
  which 
  the 
  spine 
  develops 
  as 
  the 
  

   side 
  flattens, 
  first, 
  into 
  a 
  rib 
  and 
  spine, 
  and, 
  lastly, 
  into 
  a 
  rib 
  and 
  

   two 
  spines. 
  

  

  Tt 
  is 
  from 
  the 
  earlier 
  stage 
  of 
  this 
  last 
  group 
  that 
  I 
  imagine 
  the 
  

   Sonninia 
  -branch 
  to 
  have 
  sprung 
  — 
  a 
  stage 
  when 
  the 
  coronet 
  of 
  single 
  

   spines 
  was 
  present, 
  as 
  well 
  as 
  the 
  carina. 
  I 
  imagine 
  that 
  the 
  

   Sonninia-ancestor 
  persisted 
  in 
  the 
  coronet 
  of 
  single 
  spines 
  and 
  even 
  

   exaggerated 
  the 
  character 
  ; 
  and 
  then 
  that 
  it 
  added 
  sulci 
  each 
  side 
  

   of 
  the 
  carina 
  in 
  the 
  same 
  manner 
  as 
  Pleuroceras 
  had 
  done 
  at 
  a 
  

   later 
  period 
  of 
  its 
  life. 
  In 
  fact 
  the 
  Sonninia-bi&neh. 
  accompanied 
  

   Pleuroceras 
  until 
  the 
  latter 
  fell 
  away 
  and 
  flattened 
  its 
  side 
  and 
  de- 
  

   veloped 
  its 
  single 
  spine 
  into 
  a 
  rib 
  and 
  two 
  spines. 
  Then 
  the 
  Son- 
  

   mnm-branch 
  remained 
  constant 
  to 
  the 
  old 
  shape, 
  and 
  apparently 
  it 
  

   retained 
  this 
  shape 
  for 
  a 
  long 
  period. 
  Ultimately 
  it 
  narrowed 
  its 
  

   ventral 
  area, 
  decreased 
  the 
  sulcations, 
  flattened 
  its 
  side, 
  and, 
  in 
  old 
  

   age, 
  replaced 
  the 
  large 
  spines 
  by 
  very 
  coarse 
  ribs. 
  It 
  is 
  in 
  this 
  

   state 
  that 
  we 
  first 
  meet 
  with 
  it 
  as 
  Sonninia 
  acanthodes. 
  

  

  It 
  is 
  extraordinary 
  that 
  we 
  are 
  acquainted 
  with 
  no 
  species 
  of 
  

   Sonninia 
  before 
  the 
  Concavum-zone, 
  consequently 
  my 
  theory 
  con- 
  

   cerning 
  its 
  descent 
  must 
  be 
  entirely 
  derived 
  from 
  a 
  study 
  of 
  the 
  

   inner 
  whorls 
  of 
  Sonninia 
  acanthodes. 
  I 
  will 
  briefly 
  trace 
  the 
  life- 
  

   history 
  of 
  a 
  specimen 
  as 
  shown 
  by 
  its 
  inner 
  whorls 
  : 
  — 
  

  

  1. 
  Diameter 
  | 
  line. 
  A 
  smooth, 
  globose, 
  rather 
  involute, 
  thick 
  

   shell, 
  in 
  form 
  not 
  unlike 
  Nannites 
  fugacc 
  (Zittel, 
  ' 
  Uandbuch 
  Palaont.' 
  

   " 
  Ceph." 
  p. 
  446), 
  but 
  with 
  a 
  larger 
  umbilicus 
  (fig. 
  22 
  ; 
  compare 
  also 
  

   fig. 
  24). 
  Suture 
  simple, 
  Goniatite 
  style 
  (fig. 
  6). 
  

  

  2. 
  Diameter 
  1 
  line. 
  Yentral 
  area 
  arched, 
  with 
  carina 
  and 
  two 
  

   small 
  sulci 
  ; 
  coronet 
  of 
  spines 
  first 
  commencing 
  (fig. 
  23, 
  a, 
  b). 
  

  

  3. 
  Diameter 
  2 
  lines. 
  Yentral 
  area 
  more 
  flattened, 
  with 
  carina 
  

   and 
  two 
  sulci 
  (fig. 
  23, 
  c). 
  

  

  4. 
  Diameter 
  4 
  J 
  lines. 
  Yentral 
  area 
  becoming 
  more 
  arched, 
  sulci 
  

   present, 
  but 
  not 
  so 
  conspicuous 
  proportionally 
  * 
  ; 
  carina 
  hollow 
  ; 
  

   ribs 
  with 
  forward 
  inclination 
  from 
  spines 
  towards 
  sulci. 
  

  

  The 
  first 
  three 
  of 
  the 
  above 
  correspond 
  in 
  every 
  way 
  with 
  the 
  

   changes 
  observable 
  in 
  the 
  inner 
  whorls 
  of 
  Pleuroceras 
  spinatum, 
  &c. 
  

   Number 
  4 
  corresponds 
  practically 
  in 
  most 
  respects, 
  but 
  differs 
  in 
  

   the 
  decrease 
  of 
  the 
  sulci, 
  while 
  the 
  spines 
  have 
  become 
  more 
  

   prominent. 
  

  

  The 
  annexed 
  Table 
  (p. 
  655) 
  is 
  intended 
  to 
  represent 
  the 
  manner 
  in 
  

   which 
  Sonninia 
  is 
  related 
  to 
  Pleuroceras 
  and 
  Amaltheus, 
  in 
  accordance 
  

   with 
  these 
  developments. 
  

  

  The 
  most 
  remarkable 
  point 
  in 
  the 
  Table 
  is 
  the 
  absence 
  of 
  all 
  species 
  

   from 
  the 
  Falciferum- 
  to 
  the 
  Murchisonce-zone, 
  and 
  yet 
  that 
  we 
  

   should 
  find 
  in 
  the 
  Concavum-zone 
  an 
  undoubted 
  Pleuroceras. 
  In 
  the 
  

  

  * 
  In 
  the 
  adult 
  Sonninia 
  acanthodes 
  these 
  sulci 
  are 
  almost 
  obsolete. 
  Still 
  the 
  

   rudimentary 
  sulci 
  are 
  very 
  important, 
  because 
  they 
  are 
  marks 
  of 
  descent 
  from 
  

   a 
  sulcate 
  ancestor. 
  They 
  are 
  persistent 
  in 
  the 
  adults 
  of 
  all 
  the 
  less-developed 
  

   SonninicB 
  — 
  those 
  with 
  broad 
  ventral 
  area; 
  but 
  are 
  absent 
  in 
  the 
  more 
  highly- 
  

   developed 
  forms, 
  Sonn. 
  Sowerbyi, 
  propinquans, 
  &c. 
  

  

  