﻿662 
  ME. 
  S. 
  S. 
  BUCKMAN 
  01* 
  THE 
  DESCENT 
  OF 
  

  

  which 
  in 
  turn 
  gradually 
  become 
  less 
  prominent 
  ; 
  but 
  the 
  secondary 
  

   ribs 
  continue 
  without 
  alteration 
  to 
  a 
  very 
  late 
  period. 
  

  

  This 
  species 
  is 
  remarkable 
  for 
  its 
  apparent 
  resemblance 
  to 
  species 
  

   of 
  Sonninia 
  such 
  as 
  Sonn. 
  adicra 
  and 
  Sonn. 
  Soiverbyi, 
  especially 
  the 
  

   form 
  figured 
  by 
  d'Orbigny 
  (Pal. 
  Franc, 
  pi. 
  119). 
  In 
  my 
  own 
  col- 
  

   lection 
  a 
  large 
  specimen 
  lay 
  under 
  that 
  name 
  for 
  many 
  years 
  ; 
  while 
  

   in 
  the 
  collection 
  of 
  the 
  late 
  Dr. 
  Wright 
  I 
  noticed 
  a 
  specimen 
  of 
  this 
  

   species 
  labelled 
  " 
  Harp. 
  Sowerbii." 
  From 
  any 
  specimens 
  of 
  Sonninia 
  

   this 
  species 
  is 
  distinguished 
  by 
  its 
  suture-line 
  (figs. 
  17, 
  18), 
  and 
  by 
  its 
  

   ribs 
  meeting 
  the 
  carina 
  at 
  right 
  ang]es 
  in 
  youth, 
  and 
  with 
  only 
  a 
  

   slight 
  forward 
  sweep 
  when 
  adult. 
  

  

  There 
  are 
  two 
  forms 
  of 
  this 
  species 
  in 
  my 
  cabinet. 
  

  

  1. 
  Large 
  coarse 
  spines 
  and 
  elliptical 
  aperture, 
  with 
  acute 
  ventral 
  

   area 
  (the 
  form 
  figured 
  by 
  Yacek). 
  

  

  2. 
  Spines 
  closer, 
  smaller 
  and 
  more 
  numerous 
  ; 
  in 
  other 
  respects 
  

   similar 
  to 
  1. 
  In 
  mature 
  age 
  the 
  spines 
  are 
  coarse 
  ; 
  but 
  in 
  old 
  

  

  "age 
  they 
  disappear, 
  while 
  the 
  ventral 
  area 
  is 
  flatter 
  and 
  the 
  whorls 
  

   are 
  more 
  subquadrate 
  in 
  shape. 
  

  

  To 
  Hammatoceras 
  subinsigne 
  * 
  this 
  species 
  bears 
  very 
  great 
  resem- 
  

   blance 
  ; 
  but 
  it 
  has 
  a 
  wider 
  umbilicus, 
  and 
  has 
  its 
  spines 
  more 
  nearly 
  

   upon 
  the 
  middle 
  of 
  the 
  whorl 
  and 
  persistent 
  until 
  a 
  much 
  later 
  date. 
  

   Looking 
  at 
  these 
  facts 
  I 
  am 
  doubtful 
  if 
  it 
  can 
  be 
  a 
  mutation 
  of 
  Hamm. 
  

   subinsigne, 
  and 
  it 
  seems 
  too 
  thick 
  for 
  a 
  mutation 
  of 
  H. 
  Lorteti 
  

   (Dum.). 
  

  

  The 
  horizon 
  of 
  this 
  species 
  is 
  the 
  MurcJiisonce-zone. 
  Possibly 
  

   some 
  specimens 
  may 
  have 
  come 
  from 
  the 
  Concavum-zone 
  ; 
  but 
  the 
  

   evidence 
  is 
  not 
  conclusive. 
  

  

  Bradford 
  Abbas 
  and 
  Half-way 
  House, 
  Dorset, 
  are 
  the 
  localities 
  

   which 
  have 
  supplied 
  the 
  specimens 
  in 
  my 
  cabinet. 
  

  

  Hammatoceras 
  amplectens, 
  n. 
  sp. 
  

  

  Discoidal, 
  compressed, 
  highly 
  involute. 
  Whorls 
  very 
  broad, 
  almost 
  

   smooth, 
  ornamented 
  with 
  short 
  ribs 
  on 
  the 
  outer 
  area, 
  and 
  some 
  faint 
  

   undulations 
  at 
  intervals 
  on 
  the 
  side. 
  Ventral 
  area 
  barely 
  defined, 
  sloping 
  

   and 
  ribbed, 
  rounded 
  where 
  the 
  test 
  is 
  absent, 
  but 
  otherwise 
  ornamented 
  

   with 
  a 
  small 
  carina, 
  possibly 
  degraded 
  from 
  the 
  hollow 
  type. 
  In- 
  

   clusion 
  covers 
  the 
  whole 
  whorl, 
  except 
  when 
  the 
  body-chamber 
  is 
  

   present, 
  and 
  then 
  the 
  customary 
  recession 
  of 
  the 
  inner 
  margin 
  takes 
  

   place. 
  Umbilicus 
  small 
  and 
  rather 
  deep. 
  Aperture 
  subtriangularf. 
  

  

  This 
  species 
  is 
  the 
  next 
  descendant 
  from 
  Hamm. 
  amaltheiforme 
  

   (Yacek), 
  of 
  which 
  it 
  is 
  an 
  actual 
  mutation. 
  Hamm. 
  amaltheiforme 
  

   was 
  placed 
  by 
  Yacek 
  in 
  Harpoceras 
  ; 
  but 
  it 
  is 
  a 
  genuine 
  descendant 
  

   of 
  Hamm. 
  insigne, 
  and 
  only 
  differs 
  from 
  other 
  Hammatocerata 
  by 
  com- 
  

  

  * 
  I 
  take 
  Duruortier's 
  figures 
  (Etudes 
  pal. 
  Bassin 
  Bhone, 
  iv. 
  pi. 
  liii.) 
  as 
  the 
  

   type, 
  since 
  they 
  are 
  the 
  earliest 
  figures 
  of 
  the 
  species. 
  

  

  t 
  The 
  figure 
  of 
  Parkinsonia 
  wurttemburgica 
  given 
  by 
  Bayle 
  (Explic. 
  Carte 
  

   geol. 
  France, 
  pi. 
  lxix. 
  fig. 
  1 
  only) 
  represents 
  exactly 
  the 
  side 
  view 
  of 
  this 
  species. 
  

   This 
  is 
  a 
  remarkable 
  instance 
  of 
  senile 
  convergence. 
  Compare 
  also 
  Amm. 
  Par- 
  

   jkinsoni 
  compressus, 
  Quenstedt 
  (Amm. 
  Schwabischen 
  Jura, 
  pi. 
  lxxii. 
  figs. 
  12 
  & 
  15), 
  

   from 
  which 
  our 
  species 
  differs, 
  outwardly, 
  in 
  having 
  a 
  carina 
  on 
  the 
  ventral 
  area 
  

   instead 
  of 
  a 
  smooth 
  space. 
  

  

  