﻿658 
  MK. 
  S. 
  S. 
  BTTCEMAN 
  ON 
  THE 
  DESCENT 
  OF 
  

  

  disposition 
  and 
  proportion 
  of 
  the 
  lobes 
  and 
  saddles, 
  while 
  differing 
  

   only 
  in 
  minor 
  points 
  ; 
  the 
  same 
  deep 
  superior 
  lateral 
  saddle 
  is 
  notice- 
  

   able 
  in 
  both 
  species 
  (figs. 
  2 
  & 
  3). 
  

  

  Another 
  genus, 
  an 
  offspring 
  of 
  the 
  iStownwim-ancestor, 
  may 
  be 
  

   recognized 
  in 
  Witchellia. 
  It 
  bears 
  marks 
  of 
  its 
  descent, 
  especially 
  

   in 
  the 
  deeply 
  furrowed 
  ventral 
  area 
  — 
  furrowed, 
  that 
  is, 
  when 
  the 
  

   test 
  is 
  absent. 
  It 
  also 
  shows 
  its 
  descent 
  and 
  its 
  relationship 
  to 
  

   Sonninia 
  by 
  the 
  occasional 
  production 
  of 
  sharp 
  spines 
  on 
  the 
  inner 
  

   whorls. 
  However, 
  specimens 
  which 
  show 
  these 
  spines 
  are 
  certainly 
  

   rare 
  *. 
  The 
  appearance 
  of 
  spines 
  must 
  be 
  regarded 
  as 
  a 
  case 
  of 
  re- 
  

   version, 
  and 
  since 
  it 
  is 
  not 
  accompanied 
  by 
  any 
  other 
  difference 
  from 
  

   the 
  normal 
  form, 
  the 
  feature 
  cannot 
  be 
  considered 
  even 
  of 
  specific 
  

   value. 
  Another 
  point 
  in 
  which 
  Witcliellia 
  indicates 
  its 
  descent 
  

   is 
  by 
  the 
  long 
  ventral 
  projection 
  of 
  the 
  ribs 
  — 
  a 
  character 
  seen 
  in 
  

   Amaltheus 
  and 
  Pleuroceras, 
  though 
  not 
  so 
  conspicuous 
  in 
  Sonninia. 
  

   The 
  suture-line 
  may 
  be 
  described 
  as 
  a 
  Sonninia- 
  suture-line 
  with 
  

   broad-stemmed 
  lobes, 
  or 
  as 
  a 
  Pleuroceras-sutuie-line 
  which 
  has 
  

   acquired 
  auxiliary 
  lobes 
  and 
  more 
  ornamentation 
  in 
  accordance 
  with 
  

   an 
  expanded 
  side. 
  (Pigs. 
  11, 
  12.) 
  

  

  A 
  consideration 
  of 
  the 
  suture-line 
  and 
  the 
  presence 
  of 
  small 
  spines 
  

   leads 
  me 
  to 
  the 
  conclusion 
  that 
  Witchellia 
  parted 
  from 
  the 
  Sonninia- 
  

   parent-stem 
  before 
  the 
  birth 
  of 
  the 
  S 
  ' 
  onninia-acanthodes-i 
  brm. 
  

  

  The 
  least 
  developed 
  species 
  of 
  Witcliellia 
  is 
  undoubtedly 
  the 
  

   evolute 
  W. 
  Sutneri 
  (Branco). 
  By 
  various 
  intermediate 
  — 
  undescribed 
  

   — 
  species, 
  each 
  acquiring 
  a 
  smaller 
  umbilicus, 
  a 
  broader 
  whorl, 
  and 
  

   losing 
  their 
  ribs 
  earlier 
  in 
  life, 
  it 
  is 
  connected 
  with 
  or 
  develops 
  into 
  

   Witcliellia 
  Iceviuscula 
  (Sowerby). 
  One 
  of 
  the 
  intermediate 
  species 
  

   attains 
  11 
  inches 
  in 
  diameter 
  ; 
  but 
  W. 
  Iceviuscula 
  decreases 
  to 
  4| 
  

   inches, 
  and 
  sometimes 
  loses 
  its 
  ribs 
  at 
  one 
  inch. 
  A 
  further 
  mutation 
  

   of 
  W. 
  Iceviuscula 
  loses 
  the 
  distinctive 
  ventral 
  furrows 
  when 
  adult. 
  

  

  Conclusions. 
  

  

  1. 
  The 
  genus 
  Sonninia 
  and 
  other 
  cognate 
  genera 
  are 
  correctly 
  

   separated 
  from 
  the 
  Hildoceratidse, 
  i. 
  e. 
  the 
  descendants 
  of 
  Arietites, 
  

   and 
  also 
  from 
  Hammatoceras 
  and 
  its 
  allies, 
  i. 
  e. 
  descendants 
  of 
  

   Deroceras. 
  

  

  2. 
  The 
  genus 
  Sonninia 
  and 
  other 
  cognate 
  genera 
  can 
  be 
  either 
  

   included 
  directly 
  in 
  the 
  Family 
  Amaltheidse, 
  or 
  can 
  be 
  classed 
  in 
  a 
  

   subfamily 
  thereof. 
  

  

  Descriptions 
  of 
  New 
  Sjiecies. 
  

   Sonninia 
  acanhtodes, 
  n. 
  sp. 
  (PI. 
  XXII. 
  figs. 
  6, 
  7, 
  22, 
  23.) 
  

  

  Discoidal, 
  evolute, 
  hollo 
  w-carinate. 
  Whorls 
  circular, 
  ornamented 
  

   with 
  arcuate 
  ribs 
  projected 
  ventrally, 
  and 
  with 
  large 
  spines 
  

   on 
  the 
  middle 
  of 
  the 
  lateral 
  area 
  at 
  irregular 
  intervals. 
  Ventral 
  

   area 
  not 
  defined 
  : 
  carina 
  hollow, 
  not 
  very 
  prominent, 
  on 
  the 
  core 
  

  

  * 
  It 
  has 
  come 
  to 
  pass 
  that 
  the 
  spinous 
  stage 
  is 
  omitted 
  on 
  account 
  of 
  the 
  

   early 
  inheritance 
  of 
  later 
  stages. 
  Omissions 
  of 
  this 
  kind 
  have 
  been 
  frequently 
  

   noted 
  by 
  Prof. 
  Hyatt. 
  

  

  