﻿DK. 
  COOKK 
  : 
  OPKRCULUM 
  OF 
  THE 
  GKNUS 
  BVliSA. 
  9 
  

  

  B. 
  Sub-genus 
  Aspa. 
  

  

  Section 
  Aspa. 
  

  

  margmata, 
  Gmel. 
  (tlie 
  only 
  species). 
  Operculum 
  thin, 
  semi- 
  

   transparent, 
  lines 
  of 
  growth 
  clearly 
  marked. 
  Shape 
  as 
  in 
  Bursa, 
  

   hut 
  not 
  so 
  triaiiijular, 
  nucleus 
  similarly 
  situated, 
  slightly 
  more 
  

   removed 
  from 
  the 
  left 
  margin. 
  

  

  C. 
  Sub-genus 
  Bufonaria. 
  

  

  {a) 
  Section 
  Crossata. 
  

  

  General 
  characteristics 
  : 
  operculum 
  ovate, 
  rounded 
  at 
  both 
  ends, 
  

   thick, 
  dark 
  horn-colour, 
  lines 
  of 
  growth 
  obscure, 
  a 
  nucleus 
  indicated 
  

   but 
  not 
  existing, 
  lateral, 
  sub-terminal, 
  to 
  right. 
  

  

  1. 
  califurnica, 
  Hinds. 
  Operculum 
  as 
  in 
  tlie 
  section. 
  In 
  a 
  remark- 
  

   ably 
  fine 
  and 
  clear 
  specimen 
  from 
  McA. 
  there 
  is 
  almost 
  a 
  visible 
  

   nucleus 
  on 
  the 
  margin 
  low 
  down 
  on 
  the 
  right. 
  

  

  2. 
  ventricosa, 
  Brod. 
  Operculum 
  a 
  little 
  wider 
  than 
  in 
  californica, 
  

   hut 
  in 
  other 
  respects 
  generally 
  corresponding. 
  

  

  [h) 
  Section 
  Bufonaria 
  { 
  — 
  lutufa, 
  Jouss.). 
  

  

  1. 
  rulela, 
  Bolt. 
  ( 
  = 
  ^rt/»/>««, 
  auctt.). 
  Operculum 
  sub-ovate, 
  not 
  very 
  

   thick, 
  rounded 
  at 
  top, 
  angled 
  below, 
  nucleus 
  distinct, 
  sub-lateral, 
  to 
  

   right, 
  well 
  away 
  from 
  margin, 
  below 
  the 
  centre, 
  lines 
  of 
  growth 
  

   well 
  marked, 
  but 
  not 
  raised 
  in 
  ridges. 
  

  

  2. 
  scrobiculator, 
  L. 
  Operculum 
  sub-ovate, 
  rather 
  thin, 
  semi- 
  

   transparent, 
  rounded 
  above 
  and 
  below, 
  nucleus 
  distinct, 
  sub-lateral, 
  

   to 
  right, 
  well 
  away 
  from 
  margin, 
  below 
  centre, 
  lines 
  of 
  growth 
  well 
  

   marked, 
  but 
  not 
  raised 
  in 
  ridges. 
  

  

  On 
  a 
  general 
  survey 
  of 
  the 
  evidence 
  aflForded 
  by 
  the 
  operculum, 
  

   the 
  following 
  points 
  seem 
  to 
  stand 
  out 
  : 
  — 
  

  

  1. 
  There 
  is 
  a 
  large 
  group 
  of 
  Bursa 
  {Bursa, 
  s.str. 
  -j- 
  Chasmotheca 
  

   -\- 
  ? 
  Marsupina) 
  with 
  a 
  more 
  or 
  less 
  triangular 
  operculum, 
  nucleus 
  

   well 
  marked, 
  in 
  the 
  centre 
  of 
  the 
  left 
  sub-margin. 
  

  

  2. 
  Closely 
  allied 
  to 
  this 
  group 
  stands 
  the 
  sub-genus 
  Aspa, 
  

   conchologically 
  different, 
  but 
  with 
  an 
  operculum 
  of 
  similar 
  general 
  

  

  ' 
  construction. 
  

  

  3. 
  Another 
  large 
  group 
  [Ranella 
  -j- 
  Lampadopsis 
  -\- 
  Coluhrellina) 
  

   possesses 
  an 
  ovate 
  operculum, 
  witli 
  nucleus 
  to 
  the 
  ri(!}it 
  or 
  near 
  tlie 
  

   extremity, 
  generally 
  less 
  well 
  marked, 
  sub-marginal 
  or 
  sub-terminal. 
  

   Closely 
  allied 
  to 
  this 
  group, 
  and 
  possibly 
  not 
  separable 
  from 
  it, 
  are 
  

   the 
  species 
  contained 
  in 
  section 
  Bufonaria. 
  

  

  4. 
  Further 
  removed 
  comes 
  a 
  group 
  {Crossata) 
  geographically 
  

   separate 
  from 
  the 
  Bufonaria 
  section, 
  and 
  with 
  an 
  operculum 
  not 
  

   essentially 
  dissimilar 
  to 
  that 
  of 
  certain 
  species 
  of 
  Triton 
  {Lotorium). 
  

  

  As 
  Dall 
  has 
  remarked, 
  " 
  the 
  llanellas 
  shade 
  very 
  gradually 
  into 
  

   the 
  forms 
  formerly 
  known 
  as 
  Triton, 
  judged 
  by 
  most 
  of 
  their 
  

   characters," 
  and 
  it 
  is 
  interesting 
  to 
  observe 
  how, 
  on 
  the 
  one 
  hand, 
  

   Gyrineum 
  and 
  Auq>/rina 
  carry 
  the 
  " 
  Banella" 
  shape 
  of 
  shell, 
  minus 
  

   the 
  anal 
  sulcus, 
  into 
  tlie 
  " 
  Tritons 
  ", 
  while 
  the 
  shell 
  shape 
  of 
  certain 
  

  

  