﻿ANNIVERSARY 
  ADDRESS 
  OF 
  THE 
  PRESIDENT. 
  229 
  

  

  of 
  involution 
  of 
  the 
  ivhorls 
  ; 
  5th, 
  the 
  presence 
  or 
  absence 
  of 
  the 
  aptychus, 
  

   its 
  form 
  and 
  structure. 
  In 
  all 
  the 
  Ammonitidae 
  the 
  position 
  of 
  the 
  

   siphuncle 
  is 
  invariably 
  in 
  the 
  centre 
  of 
  the 
  abdomen, 
  and 
  in 
  the 
  onter 
  

   margin 
  of 
  the 
  shell 
  (back 
  or 
  heel 
  of 
  old 
  authors) 
  opposite 
  to 
  the 
  

   columella 
  or 
  dorsal 
  border. 
  The 
  spiral 
  described 
  by 
  the 
  growth 
  of 
  

   the 
  shell 
  was 
  taken 
  as 
  the 
  basis 
  for 
  the 
  establishment 
  of 
  several 
  

   genera 
  by 
  the 
  older 
  authors, 
  Lamarck, 
  Parkinson, 
  von 
  Hauer, 
  

   D'Orbigny, 
  Leveille, 
  &c. 
  No 
  less 
  than 
  16 
  genera 
  have 
  been 
  thus 
  

   characterized, 
  but 
  in 
  conjunction 
  with 
  other 
  characters 
  it 
  becomes 
  

   subordinate. 
  Prof. 
  Neumayer 
  (loc. 
  cit.) 
  proposed 
  a 
  classification 
  

   grouped 
  into 
  the 
  four 
  families 
  Arcestid^e, 
  Tropitid^:, 
  Lttoceratid^:, 
  

  

  and 
  iEGOCERATID^. 
  

  

  The 
  family 
  Arcestidae 
  includes 
  the 
  forms 
  occurring 
  abundantly 
  in 
  

   the 
  Triassic 
  strata 
  of 
  the 
  eastern 
  Alps 
  and 
  those 
  in 
  the 
  Red 
  Triassic 
  

   Alpine 
  Limestone 
  at 
  Hallstatt. 
  Only 
  one 
  genus 
  (Amaltheus) 
  of 
  this 
  

   family 
  occurs 
  in 
  Britain. 
  The 
  remaining 
  7 
  genera 
  differ 
  essentially 
  

   from 
  any 
  known, 
  either 
  in 
  the 
  Jurassic 
  or 
  Cretaceous 
  rocks. 
  

  

  The 
  members 
  of 
  the 
  second 
  family 
  (Tropitidae) 
  are 
  all 
  Triassic, 
  

   and 
  do 
  not 
  occur 
  in 
  Britain. 
  

  

  The 
  Lytoceratidae 
  include 
  many 
  diverse 
  groups, 
  especially 
  as 
  

   regards 
  their 
  external 
  form. 
  The 
  genera 
  Hamites, 
  Turrilites, 
  

   Baculites, 
  and 
  Phylloceras 
  belong 
  here, 
  all 
  occur 
  in 
  Britain 
  and 
  in 
  

   the 
  Cretaceous 
  rocks. 
  Phylloceras 
  is 
  also 
  Upper 
  and 
  Middle 
  Lias. 
  

  

  The 
  JEgoceratidae 
  include 
  many 
  groups 
  widely 
  differing 
  in 
  general 
  

   form 
  and 
  structure 
  ; 
  they 
  may 
  be 
  naturally 
  classed 
  under 
  3 
  sections, 
  

   which 
  will 
  be 
  noticed 
  hereafter. 
  

  

  Fam. 
  I. 
  Arcestid^;. 
  

  

  Contains 
  8 
  genera: 
  — 
  1. 
  Arcestes 
  ; 
  2. 
  Didymites; 
  3. 
  Lohites 
  ; 
  4. 
  

   Ptychites; 
  5. 
  Pinacoceras; 
  6. 
  Sageceras 
  ; 
  7. 
  Amaltheus; 
  8. 
  Schloen- 
  

   bachia. 
  

  

  Of 
  the 
  above 
  8 
  genera 
  Amaltheus 
  is 
  the 
  only 
  British 
  genus. 
  It 
  

   has 
  a 
  very 
  extensive 
  range 
  in 
  time, 
  as 
  it 
  occurs 
  in 
  the 
  Trias, 
  and 
  

   is 
  largely 
  developed 
  in 
  the 
  Jurassic 
  rocks, 
  commencing 
  in 
  the 
  

   Lias 
  with 
  Amal. 
  Guibalianus, 
  D'Orb., 
  Amal. 
  margaritatus, 
  Brug., 
  

   and 
  Amal. 
  spinatus, 
  Sow. 
  Many 
  species 
  also 
  occur 
  in 
  the 
  Creta- 
  

   ceous 
  rocks, 
  such 
  as 
  Amal. 
  Gervillianus, 
  D'Orb., 
  Amal. 
  Vibrayeanus, 
  

   D'Orb. 
  ; 
  the 
  well-known 
  Amal. 
  oxynotus, 
  Am. 
  Simpsoni, 
  and 
  Am. 
  

   impendens 
  are 
  all 
  Lower 
  Lias. 
  Amal. 
  Oppeli 
  is 
  from 
  the 
  same 
  

   horizon. 
  

  

  Genus 
  Schloenbachia. 
  — 
  Limited 
  to 
  the 
  Cretaceous 
  rocks 
  ; 
  in 
  

   Britain 
  the 
  old 
  group 
  Cristati 
  contains 
  the 
  species. 
  The 
  typical 
  

   form 
  is 
  Schloenbachia 
  cristata. 
  

  

  The 
  genera 
  Arcestes, 
  Didymites, 
  Lobites, 
  Ptychites, 
  Pinacoceras, 
  

   and 
  Sageceras 
  possess 
  no 
  British 
  species, 
  and 
  most 
  of 
  them 
  are 
  

   Triassic 
  genera. 
  

  

  Fam. 
  II. 
  Tropitid^}. 
  

  

  Contains 
  5 
  genera 
  : 
  — 
  1. 
  Tropxtes; 
  2. 
  Trachyceras 
  ; 
  3. 
  Choristo- 
  

   cenas 
  ; 
  4. 
  Bhabdoceras 
  ; 
  and 
  5. 
  Cochloceras. 
  

   None 
  of 
  these 
  genera 
  include 
  British 
  species. 
  

  

  