﻿656 
  MB. 
  S. 
  5. 
  BECKMAX 
  OX 
  THE 
  DESCENT 
  OE 
  

  

  face 
  of 
  this 
  fact 
  trie 
  absence 
  of 
  links 
  between 
  Sonninia 
  and 
  PU 
  

  

  cerag 
  cannot 
  be 
  nsed 
  as 
  an 
  argument 
  against 
  the 
  connexion 
  of 
  the 
  

   latter 
  with 
  the 
  former. 
  If 
  we 
  now 
  trace 
  the 
  further 
  development 
  

   of 
  Sonninia. 
  we 
  shall 
  see 
  that 
  it 
  follows 
  the 
  same 
  lines 
  which 
  had 
  

   been 
  followed 
  by 
  Amaltliev.s. 
  from 
  Arnal. 
  gloriosus 
  to 
  Ama 
  1 
  . 
  

   preestabUis. 
  It 
  is 
  true 
  that 
  we 
  shall 
  not 
  find 
  the 
  crenulated 
  carina 
  ; 
  

   but 
  I 
  cannot 
  regard 
  the 
  absence 
  of 
  this 
  as 
  disproving 
  the 
  descent 
  of 
  

   Sonninia 
  and 
  Pleuroeeras 
  from 
  a 
  common 
  ancestor. 
  "We 
  shall, 
  how- 
  

   ever, 
  rind 
  that, 
  as 
  in 
  AmaWieus. 
  the 
  spines 
  are 
  displaced 
  by 
  ribs, 
  at 
  

   first 
  irregularly, 
  and 
  afterwards 
  altogether 
  ; 
  that 
  the 
  ribs 
  become 
  

   gradually 
  less 
  prominent, 
  and 
  finally 
  give 
  place 
  to 
  smoothness 
  : 
  that 
  

   the 
  whorl 
  becomes 
  broader, 
  the 
  involution 
  greater, 
  the 
  ventral 
  area 
  

   thinner, 
  until, 
  instead 
  of 
  the 
  spinous, 
  quadrangular-whorled, 
  evolute 
  

   Sonninia 
  acantlioJ^s. 
  we 
  have 
  the 
  smooth, 
  fiat, 
  disciform 
  species 
  

   misnamed 
  Sunn, 
  corrugata*. 
  

  

  Accompanying 
  these 
  changes 
  we 
  see 
  a 
  development 
  of 
  the 
  suture- 
  

   line 
  in 
  the 
  matter 
  of 
  complexity 
  ; 
  in 
  fact, 
  we 
  see 
  the 
  same 
  develop- 
  

   ment 
  as 
  that 
  which 
  occurred 
  from 
  the 
  sutures 
  of 
  young 
  Pleuroeeras 
  

   — 
  and 
  possibly 
  younger 
  ArnaWiev.s 
  — 
  to 
  those 
  of 
  the 
  adult 
  Arnoltheus 
  

   mritatus. 
  The 
  accompanying 
  Plate 
  (PL 
  XXII. 
  ) 
  will 
  illustrate 
  

   this 
  matter. 
  

  

  Fig. 
  6 
  shows 
  the 
  suture-line 
  of 
  a 
  very 
  young 
  Sonninia 
  ; 
  it 
  re- 
  

   sembles 
  a 
  Goniatite-suture. 
  Fig. 
  7 
  depicts 
  the 
  suture 
  of 
  Sonninia 
  

   when 
  somewhat 
  older, 
  and 
  shows 
  the 
  resemblance 
  to 
  fig. 
  1, 
  the 
  

   suture 
  of 
  Pleur. 
  pseudocostatum. 
  Figs. 
  S, 
  9 
  show 
  the 
  great 
  change 
  

   which 
  has 
  taken 
  place 
  to 
  develop 
  that 
  simple 
  suture 
  into 
  such 
  a 
  com- 
  

   plex 
  one, 
  which 
  is, 
  however, 
  in 
  its 
  turn 
  comparable 
  with 
  fig. 
  -i 
  (half- 
  

   grown 
  Ama 
  1 
  .. 
  margaritatus). 
  Fig. 
  10 
  is 
  the 
  suture 
  of 
  a 
  half-grown 
  

   Sonninia 
  corrugata 
  — 
  it 
  is 
  fig. 
  9 
  developed 
  and 
  pressed 
  into 
  a 
  smaller 
  

   space. 
  In 
  complexity 
  it 
  has 
  passed 
  far 
  beyond 
  fig. 
  4, 
  and 
  is 
  com- 
  

   parable 
  only 
  with 
  the 
  adult 
  Arnal. 
  maraaritatas. 
  fig. 
  5. 
  The 
  sole 
  

   difference 
  between 
  the 
  two 
  is 
  that 
  the 
  lateral 
  lobes 
  of 
  fig. 
  10 
  are 
  

   lower 
  than 
  in 
  fig. 
  5 
  ; 
  but 
  this 
  is 
  caused 
  by 
  the 
  extra 
  length 
  of 
  the 
  

   siphonal 
  lobe 
  in 
  fig. 
  5. 
  Xow 
  in 
  Arna.l. 
  margaritaius 
  the 
  ventral 
  

   area 
  is 
  practically 
  non-existent, 
  and 
  the 
  two 
  branches 
  of 
  the 
  

   siphonal 
  lobe 
  have 
  been 
  converted 
  into 
  lateral 
  lobes. 
  As 
  soon 
  as 
  

   that 
  takes 
  place 
  we 
  get 
  an 
  increase 
  in 
  the 
  size 
  of 
  these 
  branches 
  

   because 
  they 
  have 
  to 
  support, 
  not 
  an 
  arched 
  ventral 
  area, 
  but 
  a 
  

   compressed, 
  flattened 
  side. 
  

  

  This 
  resemblance 
  of 
  the 
  sutures 
  of 
  AmaWieus 
  and 
  Sonninia 
  is 
  

   most 
  extraordinary, 
  and 
  only 
  to 
  be 
  explained 
  if 
  we 
  surmise 
  that 
  the 
  

   genera 
  are 
  descended 
  from 
  a 
  common 
  ancestor, 
  and 
  that 
  while 
  one 
  

   branch. 
  AmaWieus. 
  accelerated 
  its 
  development, 
  the 
  other 
  delayed 
  

   that 
  process 
  until 
  a 
  much 
  later 
  period. 
  

  

  The 
  supposition 
  that 
  Sonninia 
  is 
  descended 
  from 
  Hammatoeertu 
  

   or 
  from 
  Haugia 
  leaves 
  this 
  extraordinary 
  resemblance 
  quite 
  unac- 
  

   counted 
  for. 
  Figs. 
  16-20 
  show 
  the 
  suture-line 
  of 
  Hammatoecras. 
  

  

  * 
  Another 
  remarkable 
  feature 
  furnishing 
  a 
  piece 
  of 
  strong 
  evidence 
  is 
  the 
  

   appearance, 
  occasionally, 
  of 
  the 
  spiral 
  lines 
  and 
  also 
  something 
  like 
  the 
  wrinkle- 
  

   layer 
  of 
  Amaltheu.s 
  in 
  certain 
  species 
  of 
  the 
  Sonnin 
  ia 
  -branch. 
  

  

  