﻿91 
  

  

  THE 
  EADULA 
  IN 
  THAIS, 
  DRUPA, 
  MORULA, 
  CONCHOLEPAS, 
  

   CRONIA, 
  lOPAS, 
  AND 
  THE 
  ALLIED 
  GENERA. 
  

  

  Ey 
  the 
  llev. 
  Dr. 
  A. 
  H. 
  Cooke, 
  F.Z.S. 
  

  

  Read 
  8th 
  November, 
  191S. 
  

  

  TiiK 
  materials 
  for 
  the 
  following 
  paper 
  are 
  derived 
  from 
  the 
  

   collectious 
  of 
  the 
  late 
  Professor 
  Gwatkin, 
  now 
  in 
  the 
  British 
  Museum 
  

   of 
  Natural 
  History, 
  the 
  Cambridge 
  Museum 
  of 
  Zoology, 
  and 
  my 
  

   own 
  collection. 
  As 
  regards 
  synonymy 
  and 
  the 
  correct 
  naming 
  

   of 
  species 
  I 
  have 
  thought 
  it 
  best 
  to 
  make 
  but 
  few 
  rectifications, 
  

   leaving 
  the 
  specimens 
  on 
  the 
  slides 
  to 
  speak 
  for 
  themselves. 
  To 
  

   nearly 
  all 
  of 
  them 
  is 
  attached 
  the 
  locality 
  and 
  the 
  name 
  of 
  the 
  

   collector 
  from 
  whom 
  Gwatkin 
  procured 
  them. 
  

  

  Thais. 
  

  

  An 
  examination 
  of 
  the 
  radula 
  of 
  the 
  various 
  species 
  of 
  Thais 
  lends 
  

   little 
  support 
  to 
  the 
  groupings 
  of 
  that 
  genus, 
  based 
  on 
  the 
  forms 
  of 
  

   the 
  shell, 
  to 
  which 
  are 
  given 
  the 
  names 
  of 
  Stramonita, 
  Trihulus, 
  

   Pohjtropa, 
  Thalessa, 
  etc. 
  Certain 
  groupings 
  are 
  indicated, 
  but 
  not 
  

   in 
  that 
  direction. 
  

  

  The 
  normal 
  type 
  of 
  a 
  Thais 
  radula 
  exhibits 
  a 
  rhachidian 
  tooth 
  

   with 
  three 
  prominent 
  cusps, 
  of 
  which 
  the 
  central 
  is 
  almost 
  always 
  

   the 
  longest. 
  The 
  two 
  external 
  cusps 
  are 
  each 
  flanked, 
  at 
  the 
  two 
  

   upper 
  ends 
  of 
  the 
  tooth, 
  by 
  a 
  bluutish 
  knob. 
  Tliis 
  knob 
  is 
  usually 
  

   inconspicuous 
  and, 
  except 
  in 
  the 
  case 
  of 
  the 
  Nucella 
  group, 
  does 
  not 
  

   approach 
  the 
  cusps 
  in 
  length. 
  

  

  The 
  central 
  cusp 
  is 
  invariably 
  simple; 
  the 
  external 
  or 
  side 
  cusps 
  

   are 
  (1) 
  simple, 
  which 
  is 
  very 
  rarely 
  the 
  case, 
  or 
  (2) 
  furnished 
  with 
  

   a 
  single 
  small 
  denticle 
  on 
  the 
  inside, 
  i.e. 
  on 
  the 
  side 
  adjacent 
  to 
  the 
  

   central 
  cusp. 
  This 
  denticle 
  (there 
  is 
  never 
  more 
  tlian 
  one) 
  is 
  

   sometimes 
  placed 
  high 
  on 
  the 
  cusp, 
  sometimes 
  so 
  low 
  that 
  it 
  almost 
  

   seems 
  to 
  be 
  separate 
  from 
  it. 
  The 
  denticle 
  is 
  usually 
  sharp 
  and 
  

   thorn-like, 
  but 
  quite 
  simple; 
  occasionally, 
  as 
  in 
  ^?f/(?, 
  melones, 
  it 
  is 
  

   somewhat 
  irregular 
  in 
  outline; 
  capensis 
  exhibits 
  the 
  extreme 
  of 
  

   variation 
  in 
  this 
  direction. 
  

  

  The 
  space 
  between 
  each 
  external 
  cusp 
  and 
  the 
  knob 
  which 
  ends 
  

   the 
  upper 
  margin 
  is 
  sometimes 
  quite 
  simple, 
  e.g. 
  in 
  ca^Jensis, 
  echinata, 
  

   a.\i(\. 
  gemmulata. 
  As 
  a 
  rule, 
  however, 
  it 
  is 
  occupied 
  by 
  a 
  number 
  of 
  

   minute 
  narrow 
  blunt 
  denticles, 
  which 
  are 
  sometimes 
  few, 
  or 
  so 
  

   small 
  as 
  to 
  be 
  scarcely 
  discernible, 
  sometimes 
  numerous 
  and 
  very 
  

   conspicuous. 
  Every 
  gradation 
  of 
  form 
  is 
  found 
  in 
  these 
  denticles, 
  in 
  

   the 
  various 
  species, 
  e.g. 
  in 
  deltoidea 
  there 
  are 
  about 
  three, 
  scarcely 
  

   visible 
  with 
  a 
  high 
  power, 
  more 
  like 
  grooves 
  or 
  wrinkles 
  on 
  the 
  

   surface 
  than 
  actual 
  separate 
  denticles, 
  while 
  e.g. 
  in 
  hamastoma, 
  

   persica, 
  riidolphi 
  there 
  are 
  four, 
  five, 
  or 
  six 
  prominent 
  and 
  well 
  

   marked. 
  In 
  species 
  where 
  these 
  denticles 
  are 
  multiplied 
  the 
  number 
  

   of 
  them 
  is 
  not 
  quite 
  constant. 
  As 
  a 
  rule 
  one 
  or 
  more 
  of 
  the 
  denticles 
  

   "climb" 
  up 
  the 
  exterior 
  surface 
  of 
  the 
  cusp, 
  which 
  has 
  the 
  effect, 
  

   under 
  a 
  high 
  power, 
  of 
  making 
  it 
  appear 
  somewhat 
  serrated; 
  this 
  

  

  