﻿232 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  Sect. 
  3. 
  Armati. 
  — 
  An 
  important 
  section 
  in 
  Britain 
  and 
  Germany. 
  

   The 
  ribs 
  on 
  the 
  sides 
  of 
  the 
  shell 
  develop 
  tubercles 
  or 
  long 
  spines 
  

   in 
  many 
  species. 
  This 
  section 
  is 
  almost 
  entirely 
  Middle 
  Lias, 
  but 
  

   JEg. 
  Birchii 
  and 
  JEg. 
  pJanicosta 
  characterize 
  the 
  zone 
  of 
  Arietites 
  

   Buchlandi 
  in 
  the 
  Lower 
  Lias, 
  whereas 
  JEg. 
  Davcei, 
  JEg. 
  armatum, 
  

   JEg. 
  Taylori, 
  JEg. 
  densinodus, 
  JEg. 
  hrevispinum, 
  JEg. 
  Valdani, 
  and 
  

   JEg. 
  Maugenesti 
  are 
  all 
  British 
  Middle-Lias 
  species, 
  and 
  from 
  the 
  

   zone 
  of 
  JEg. 
  Jamesoni, 
  Sow. 
  

  

  Sect. 
  4. 
  lnvoluti. 
  — 
  The 
  shell 
  in 
  this 
  section 
  of 
  JEgoceras 
  is 
  highly 
  

   involute 
  ; 
  the 
  inner 
  whorls 
  are 
  almost 
  entirely 
  concealed 
  in 
  some 
  

   forms 
  ; 
  in 
  others 
  the 
  umbilicus 
  is 
  much 
  wider, 
  through 
  the 
  in- 
  

   volutions 
  being 
  less. 
  The 
  zone 
  of 
  JEg. 
  Henley 
  i 
  yields 
  the 
  fol- 
  

   lowing 
  species 
  : 
  — 
  JEg. 
  striatum, 
  JEg. 
  Bechei, 
  JEg. 
  pettos, 
  and 
  JEg. 
  

   heterogenes, 
  Y. 
  & 
  B. 
  The 
  species 
  are 
  essentially 
  Middle 
  Lias 
  and 
  

   British. 
  The 
  first 
  appearance 
  of 
  this 
  section 
  is 
  in 
  the 
  Muschelkalk 
  ; 
  

   it 
  became 
  extinct 
  in 
  the 
  Middle 
  Lias. 
  

  

  Genus 
  Arietites 
  (the 
  group 
  "Arietes" 
  of 
  Von 
  Buch, 
  highly 
  

   characteristic 
  of 
  the 
  Lower 
  Lias, 
  and 
  in 
  which 
  we 
  know 
  of 
  at 
  least 
  

   20 
  species). 
  — 
  Waagen 
  established 
  this 
  genus 
  {Arietites) 
  for 
  those 
  

   species 
  having 
  a 
  wide 
  and 
  open 
  umbilicus, 
  the 
  sides 
  of 
  tho 
  shell 
  also 
  

   possessing 
  straight 
  simple 
  ribs, 
  which 
  often 
  bear 
  tubercles 
  near 
  the 
  

   ventral 
  angle, 
  as 
  in 
  A. 
  rotiformis. 
  The 
  keel 
  is 
  large 
  and 
  promi- 
  

   nent, 
  having 
  on 
  each 
  side 
  a 
  deep 
  lateral 
  channel 
  or 
  groove. 
  The 
  

   following 
  20 
  important 
  species 
  belong 
  to 
  the 
  genus 
  Arietites 
  : 
  — 
  

  

  Arietites 
  Bucklandi, 
  Sow. 
  

  

  Bodleyi. 
  Buckm. 
  

  

  Bonnardi, 
  If 
  Orb. 
  

  

  Cro9sii, 
  Wright. 
  

  

  obtusus, 
  Sow. 
  

  

  semicostatus, 
  Simp. 
  

  

  bisulcatus, 
  Brug. 
  

  

  raricostatus, 
  Ziet. 
  

  

  multicostatus, 
  Sow. 
  

  

  Collenoti, 
  D'Orb. 
  

  

  Arietites 
  rotiformis, 
  Sow. 
  

  

  Sauzeauus, 
  D'Orb. 
  

  

  stellaris, 
  Sow. 
  

  

  Scipionianus, 
  B'Orb. 
  

  

  impendens, 
  Y. 
  Sf 
  B. 
  

  

  Conybeari, 
  Sow. 
  

  

  Brookii, 
  Sow. 
  

  

  Turneri, 
  Sow. 
  

  

  obesulus, 
  Blake. 
  

  

  Nodotianus, 
  D' 
  Orb. 
  

  

  No 
  genus 
  of 
  Ammonites 
  in 
  the 
  British 
  rocks 
  is 
  more 
  important 
  

   stratigraphically. 
  None 
  of 
  the 
  species 
  pass 
  the 
  horizon 
  of 
  the 
  Lower 
  

   Lias 
  ; 
  and 
  they 
  essentially 
  characterize 
  its 
  lower 
  members. 
  12 
  of 
  

   the 
  20 
  species 
  occur 
  in 
  the 
  cliff-section 
  at 
  Lyme 
  Kegis 
  ; 
  few 
  sections 
  

   are 
  opened 
  in 
  the 
  Lower 
  Lias 
  in 
  which 
  they 
  are 
  not 
  met 
  with. 
  

  

  Subfam. 
  2. 
  Harpoceratites. 
  

  

  Genus 
  Harpoceras. 
  — 
  This 
  is 
  in 
  part 
  equivalent 
  to 
  the 
  group 
  

   Falciferi 
  of 
  Von 
  Buch, 
  which 
  in 
  Britain 
  attains 
  its 
  highest 
  de- 
  

   velopment 
  in 
  the 
  Upper 
  Lias, 
  equalling 
  in 
  stratigraphical 
  im- 
  

   portance 
  the 
  genus 
  Arietites 
  in 
  the 
  Lower 
  Lias. 
  Harpoceras 
  first 
  

   appears 
  in 
  the 
  Middle 
  Lias 
  with 
  JEgoceras 
  Jamesoni 
  in 
  3 
  well- 
  

   determined 
  species 
  — 
  H. 
  impendens, 
  H. 
  Normanianum, 
  and 
  H. 
  arie- 
  

   tiforme. 
  The 
  two 
  first-named 
  are 
  British 
  species. 
  Harpoceras 
  

   occurs 
  also 
  in 
  the 
  Middle 
  and 
  Lower 
  Jurassic 
  rocks, 
  associated 
  with 
  

  

  