﻿b 
  PHOCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIKTT. 
  

  

  Acanthina 
  tnuricata, 
  Brod,, 
  for 
  its 
  type, 
  A. 
  grandis, 
  Gray, 
  being 
  

   associated 
  with 
  it. 
  

  

  II. 
  The 
  Northeen 
  Group. 
  

   Paucilirata, 
  Stearns 
  

   Punctulata, 
  Sowb. 
  

   Vnicarinata, 
  Sowb. 
  

  

  All 
  three 
  species 
  exhibit 
  the 
  same 
  type 
  of 
  radula, 
  with 
  but 
  little 
  

   modification 
  (Figs. 
  3, 
  4). 
  The 
  central 
  cusp 
  of 
  the 
  rhachidian 
  is 
  long, 
  

   sharp, 
  and 
  thick, 
  and 
  is 
  sunk 
  in 
  a 
  sort 
  of 
  shallow 
  pit 
  between 
  the 
  

   two 
  side 
  cusps; 
  it 
  is 
  mounted 
  on 
  a 
  thick 
  and 
  rounded 
  pillar, 
  which 
  

   runs 
  right 
  through 
  to 
  the 
  lower 
  base 
  of 
  the 
  tooth, 
  in 
  which 
  its 
  end 
  

   forms 
  a 
  rounded 
  projection. 
  Thus 
  the 
  base 
  is 
  doubly 
  curved, 
  with 
  

   this 
  projection 
  between 
  the 
  curves. 
  The 
  side 
  cusps 
  are 
  broad, 
  

   sharply 
  pointed, 
  denticled 
  strongly 
  on 
  the 
  inside 
  and 
  less 
  strongly 
  on 
  

   the 
  outside; 
  they 
  are 
  mounted 
  on 
  a 
  sort 
  of 
  thickened 
  and 
  rounded 
  

   buttress, 
  not 
  so 
  thick 
  as 
  the 
  pillar 
  of 
  the 
  central 
  cusp, 
  and 
  not 
  

   running 
  through 
  to 
  the 
  base. 
  Tlie 
  knob 
  is 
  prominent 
  and 
  elevated, 
  

   and 
  sometimes 
  there 
  are 
  signs 
  of 
  an 
  additional 
  small 
  denticle 
  between 
  

   it 
  and 
  the 
  external 
  denticle- 
  of 
  the 
  side 
  cusp. 
  

  

  These 
  features 
  constitute 
  a 
  very 
  wide 
  distinction 
  between 
  this 
  

   type 
  of 
  radula 
  and 
  that 
  of 
  Thais 
  proper 
  (Fig. 
  2), 
  in 
  which 
  the 
  three 
  

   cusps, 
  whether 
  denticled 
  or 
  not, 
  form 
  simple 
  knife 
  edges 
  on 
  the 
  upper 
  

   margin 
  of 
  the 
  tooth, 
  and 
  have 
  no 
  deep-set 
  pilhir 
  or 
  buttress 
  on 
  

   which 
  they 
  are 
  mounted. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  type 
  of 
  radula 
  now 
  described 
  as 
  present 
  in 
  

   these 
  northern 
  Acanthinse 
  is 
  precisely 
  that 
  of 
  the 
  genus 
  JS'ucella. 
  

   All 
  the 
  West 
  American 
  "purpuroid" 
  forms 
  known 
  to 
  me, 
  Iwia, 
  

   Mart., 
  plicata, 
  Mart., 
  emarginata, 
  Desh., 
  and 
  their 
  varieties, 
  witli 
  

   our 
  own 
  laptllus, 
  L. 
  (Fig. 
  5), 
  exhibit 
  a 
  radula 
  of 
  this 
  type, 
  the 
  

   different 
  species 
  differing 
  only 
  in 
  minor 
  details. 
  Thus 
  the 
  northern 
  

   group 
  of 
  Acanthina, 
  from 
  the 
  point 
  of 
  view 
  of 
  the 
  radula, 
  stands 
  in 
  

   very 
  close 
  relation 
  to 
  Nucella, 
  from 
  which 
  it 
  is 
  a 
  probable 
  derivative. 
  

   AcANTHiNUCELLA, 
  ii.subgen., 
  is 
  now 
  proposed 
  by 
  me 
  for 
  the 
  reception 
  

   of 
  these 
  northern 
  Acanthina, 
  having 
  A. 
  punctulata, 
  Sowb., 
  as 
  the 
  type. 
  

  

  III. 
  The 
  Southern 
  Group. 
  

  

  A. 
  calcar, 
  Martyn, 
  with 
  its 
  closely 
  related 
  species, 
  or 
  vaiiety, 
  

   unicornis, 
  Brug. 
  (= 
  crassilabrum, 
  Lam.). 
  

  

  In 
  each 
  of 
  these 
  forms 
  the 
  rhachidian 
  tooth 
  differs 
  from 
  that 
  of 
  the 
  

   northern 
  group 
  in 
  the 
  following 
  points. 
  

  

  1. 
  The 
  central 
  cusp, 
  which 
  is 
  much 
  broader, 
  and 
  not 
  so 
  thick, 
  is 
  

   not 
  sunk 
  in 
  a 
  shallow 
  pit 
  between 
  the 
  side 
  cusps. 
  

  

  2. 
  It 
  is 
  not 
  mounted 
  on 
  a 
  strong 
  pillar 
  carried 
  through 
  to 
  the 
  

   lower 
  side 
  of 
  the 
  base, 
  but 
  its 
  roots 
  are 
  just 
  sufficiently 
  carried 
  

   through 
  to 
  cause 
  a 
  slight 
  projection 
  in 
  the 
  base-line. 
  

  

  3. 
  The 
  side 
  cusps 
  are 
  not 
  buttressed. 
  

  

  4. 
  The 
  knob 
  appears 
  to 
  be 
  doubled, 
  because 
  the 
  plate 
  which 
  

   carries 
  both 
  the 
  cusps 
  and 
  the 
  usual 
  knob 
  is 
  superposed 
  on 
  a 
  support 
  

   or 
  framework 
  which 
  is 
  also 
  sharply 
  knobbed 
  at 
  the 
  two 
  upper 
  angles. 
  

  

  Troschel 
  (Das 
  Gebiss, 
  ii, 
  pi. 
  xiii, 
  figs. 
  7, 
  8) 
  gives 
  what 
  are 
  in 
  some 
  

  

  