﻿464 
  MR. 
  S. 
  S. 
  BUCKMAX 
  ON 
  THE 
  

  

  I 
  have 
  tabulated 
  herein 
  96 
  species 
  of 
  this 
  family, 
  and 
  I 
  believe 
  

   that, 
  with 
  very 
  few 
  exceptions 
  which 
  I 
  have 
  not 
  had 
  opportuDity 
  

   to 
  study, 
  these 
  include 
  all 
  the 
  Ammonites 
  which 
  belong 
  to 
  this 
  

   family 
  in 
  the 
  zones 
  mentioned. 
  It 
  will 
  be 
  noticed 
  that, 
  of 
  all 
  these 
  

   species, 
  only 
  three 
  survive 
  the 
  period 
  known 
  as 
  the 
  Concavum-beds, 
  

   a 
  fact 
  which 
  cannot 
  but 
  be 
  regarded 
  as 
  extremely 
  striking 
  ; 
  while, 
  

   on 
  the 
  other 
  hand, 
  only 
  13 
  of 
  the 
  species 
  appeared 
  prior 
  to 
  what 
  I 
  

   propose 
  to 
  name 
  Toarcian. 
  Thus 
  we 
  are 
  left 
  with 
  80 
  species 
  of 
  

   this 
  one 
  family 
  as 
  existing 
  during 
  the 
  Toarcian 
  period 
  ; 
  and 
  we 
  

   may, 
  I 
  think, 
  with 
  propriety, 
  define 
  this 
  period 
  as 
  the 
  one 
  which 
  was 
  

   dominated 
  by 
  the 
  family 
  Hildoceratida?. 
  Curiously 
  enough, 
  different 
  

   portions 
  of 
  the 
  Toarcian 
  were 
  dominated 
  by 
  certain 
  genera 
  of 
  this 
  

   family 
  : 
  thus 
  Hildoceras, 
  Harpoceras, 
  and 
  Lillia 
  dominate 
  the 
  Falci- 
  

   ferum- 
  and 
  Commune-zones 
  ; 
  Grammoceras 
  dominates 
  the 
  Jurense- 
  

   and 
  Opalinum-zones 
  ; 
  Ludiviyia, 
  Lioceras, 
  and 
  Hyperlioceras 
  domi- 
  

   nate 
  the 
  Murcliisonce- 
  and 
  the 
  Concavum-zones. 
  

  

  It 
  would 
  seem 
  that 
  the 
  upper 
  limit 
  of 
  the 
  Toarcian 
  is 
  more 
  

   sharply 
  defined 
  upon 
  the 
  continent 
  than 
  with 
  us, 
  owing 
  to 
  the 
  

   absence, 
  apparently, 
  of 
  the 
  Concavum-beds 
  from 
  the 
  continent. 
  I 
  

   fancy 
  that 
  the 
  Concavum-zone 
  helps 
  to 
  bridge 
  over 
  the 
  hiatus 
  

   between 
  the 
  3Iur:hisonce- 
  and 
  so-called 
  Sowerbyi-zones 
  of 
  the 
  con- 
  

   tinent, 
  and 
  therefore 
  it 
  gives 
  a 
  certain 
  amount 
  of 
  difficulty. 
  It 
  is 
  

   quite 
  true 
  that 
  the 
  HildoceratidaB 
  come 
  to 
  a 
  sudden 
  end 
  within 
  the 
  

   Concavum-zone, 
  as 
  the 
  Table 
  shows 
  ; 
  but 
  we 
  also 
  meet 
  with 
  a 
  number 
  

   of 
  species 
  (as 
  yet 
  undescribed) 
  of 
  the 
  genus 
  Sonninia 
  ; 
  and 
  this 
  genus 
  

   is 
  one 
  which 
  belongs 
  especially 
  to 
  the 
  strata 
  above 
  the 
  Concavum- 
  

   zone, 
  and 
  therefore 
  helps 
  to 
  connect 
  it 
  therewith. 
  It 
  was 
  the 
  pre- 
  

   sence 
  of 
  these 
  Sonninice 
  (some 
  of 
  which 
  are 
  very 
  similar 
  to, 
  though 
  

   precursors 
  of, 
  Sonninia 
  Soiverbyi) 
  which 
  caused 
  me 
  to 
  call 
  the 
  

   Concavum-zone 
  " 
  the 
  Soiverbyi-zone 
  " 
  ; 
  especially 
  as 
  they 
  make 
  a 
  

   sudden 
  and 
  quite 
  unexpected 
  appearance 
  in 
  the 
  Concavum-zone 
  and 
  

   are 
  absent 
  from 
  the 
  Murcliisonai-zcne. 
  Probably 
  we 
  must 
  not 
  ex- 
  

   pect 
  to 
  find 
  any 
  division 
  where 
  there 
  is 
  not 
  some 
  connexion 
  with 
  

   strata 
  above 
  ; 
  and 
  the 
  value 
  of 
  these 
  Sonninice 
  is 
  again 
  balanced 
  by 
  

   Hammatoceras, 
  a 
  genus 
  which 
  began 
  in 
  the 
  Jurense-zone 
  and 
  ended, 
  

   apparently 
  rather 
  abruptly, 
  in 
  the 
  Concavum-zone, 
  having 
  been 
  

   fairly 
  persistent 
  through 
  the 
  intermediate 
  strata. 
  The 
  presence 
  of 
  

   members 
  of 
  the 
  genera 
  Oppelia 
  and 
  Lissoceras 
  is 
  evidence 
  neither 
  

   one 
  way 
  nor 
  the 
  other 
  : 
  it 
  connects 
  the 
  strata 
  no 
  more 
  with 
  the 
  

   Oppelice 
  of 
  the 
  Opalinum-zone 
  than 
  with 
  those 
  of 
  the 
  Humphrie- 
  

   sianum-zone. 
  The 
  presence 
  of 
  one 
  or 
  two 
  small 
  species 
  of 
  Steplia- 
  

   noceras 
  perhaps 
  tends 
  rather 
  to 
  unite 
  the 
  Concavum-zone 
  with 
  the 
  

   strata 
  above 
  them. 
  But 
  the 
  presence 
  of 
  Lytoceras 
  confusum 
  (a 
  

   member 
  of 
  the 
  Jurcnse- 
  group) 
  unites 
  the 
  Concavum-zone 
  very 
  forcibly 
  

   with 
  the 
  rest 
  of 
  the 
  Toarcian 
  by 
  means 
  of 
  the 
  following 
  series: 
  — 
  

   Lytoceras 
  ju 
  reuse, 
  in 
  the 
  zone 
  of 
  that 
  name; 
  Lytoceras 
  Wric/Jiti, 
  in 
  the 
  

   Opalinum-zone 
  ; 
  Lytoc. 
  amplum 
  in 
  the 
  Murcliisonce-zone 
  ; 
  because 
  we 
  

   must 
  remember 
  that 
  Lytoceras 
  Eudesianum 
  of 
  the 
  Humpliriesianum- 
  

   zone 
  does 
  net 
  belong 
  to 
  the 
  Jurense- 
  group 
  of 
  the 
  Lytocerata, 
  but 
  to 
  

   the 
  Fimbriatum-gionn. 
  After 
  all, 
  the 
  strongest 
  pakeontological 
  

  

  