﻿104 
  rKOCEIOBINGS 
  OF 
  TIIK 
  MALACOLOGICAL 
  .SOCIETY. 
  

  

  present); 
  one 
  blunt 
  tootlilet 
  lower 
  d'^wn 
  still; 
  knob 
  strong, 
  

   prominent 
  above 
  ; 
  base 
  angles 
  produced 
  ratbei 
  broadly. 
  

  

  plicata, 
  Mart. 
  : 
  Vancouver 
  I. 
  Central 
  cusp 
  long 
  ■ 
  1 
  rather 
  

   thick, 
  decpl)' 
  sunk 
  in 
  the 
  upper 
  margin 
  ; 
  side 
  cusps 
  not 
  . 
  .a 
  shorter 
  

   than 
  the 
  central, 
  with 
  a 
  rather 
  conspicuous 
  humped 
  d.nticle 
  half- 
  

   way 
  up 
  on 
  the 
  inside; 
  outside, 
  one 
  blunt 
  denticle 
  close 
  to 
  the 
  base 
  

   of 
  the 
  cusp, 
  then 
  two 
  others, 
  further 
  away 
  and 
  lower 
  down 
  ; 
  knob 
  

   elevated, 
  strong; 
  angles 
  of 
  base 
  slightly 
  produced, 
  root 
  of 
  the 
  

   central 
  cusp 
  projecting 
  sliglitly 
  in 
  the 
  middle. 
  (Fig. 
  30.) 
  

  

  emarginata, 
  Desh., 
  var. 
  saxicola, 
  auctt. 
  : 
  California. 
  This 
  form 
  

   stands 
  somewhat 
  apart 
  from 
  the 
  rest. 
  Its 
  general 
  type 
  is 
  the 
  same, 
  

   but 
  the 
  whole 
  radula 
  is 
  rather 
  small, 
  compressed, 
  and 
  narrowed 
  (the 
  

   laterals 
  are 
  reduced 
  in 
  size). 
  Central 
  cusp 
  very 
  long, 
  but 
  not 
  very 
  

   thick, 
  root 
  cari'ied 
  down 
  to 
  the 
  base, 
  but 
  not 
  projecting; 
  side 
  cusps 
  

   short, 
  and 
  narrower 
  than 
  in 
  other 
  members 
  of 
  the 
  group 
  ; 
  the 
  

   exterior 
  line 
  of 
  the 
  cusp 
  descends, 
  without 
  any 
  broadening, 
  into 
  the 
  

   mass 
  of 
  the 
  tooth, 
  one 
  strong 
  sharp 
  denticle 
  high 
  up 
  on 
  the 
  inside 
  of 
  

   tlie 
  cusp, 
  occasional 
  traces 
  of 
  a 
  tiny 
  denticle 
  low 
  down 
  outside; 
  

   knob 
  strong, 
  not 
  much 
  elevated, 
  framework 
  of 
  tooth 
  not 
  carried 
  up 
  

   so 
  high 
  from 
  knob 
  to 
  knob 
  as 
  in 
  the 
  others; 
  angles 
  of 
  base 
  

   considerably 
  more 
  produced 
  than 
  in 
  any 
  other 
  species 
  of 
  the 
  group 
  ; 
  

   base 
  arched 
  inwardly. 
  

  

  Morula, 
  Schumacher. 
  

  

  Study 
  of 
  the 
  radulaj 
  of 
  the 
  Thais 
  group 
  leads 
  one 
  to 
  think 
  that, 
  

   whereas 
  hitherto 
  a 
  great 
  deal 
  of 
  stress 
  has 
  been 
  laid 
  on 
  the 
  form, 
  

   number, 
  etc., 
  of 
  the 
  rhachidian 
  cusps, 
  the 
  shape 
  and 
  formation 
  of 
  

   the 
  mass 
  of 
  the 
  tooth, 
  mid 
  particularly 
  of 
  its 
  base, 
  has 
  an 
  equal 
  claim 
  

   on 
  our 
  notice. 
  From 
  the 
  fact 
  that 
  the 
  cusps 
  are 
  more 
  prominent, 
  

   and 
  that 
  the 
  cusps 
  of 
  one 
  tooth 
  overlie 
  tlie 
  base 
  of 
  the 
  next, 
  the 
  body 
  

   and 
  base 
  of 
  the 
  tooth 
  are 
  often 
  difficult 
  to 
  observe, 
  and 
  have, 
  in 
  

   consequence, 
  seldom 
  been 
  observed 
  at 
  all. 
  Troscliel, 
  it 
  would 
  

   appear, 
  lays 
  little 
  stress 
  on 
  this 
  part 
  of 
  his 
  subject. 
  Observation 
  

   tends 
  to 
  impress 
  upon 
  one 
  the 
  truth 
  that, 
  while 
  the 
  cusps 
  and 
  tlieir 
  

   attendant 
  denticles 
  are, 
  to 
  a 
  certain 
  extent, 
  variable 
  in 
  diffrrent 
  

   specimens 
  of 
  a 
  species, 
  and 
  even 
  in 
  the 
  same 
  sjiecimen, 
  the 
  form 
  of 
  

   the 
  base 
  remains 
  constant, 
  and 
  must 
  therefore 
  become 
  an 
  important 
  

   factor 
  in 
  our 
  estimate 
  of 
  tlie 
  radula 
  as 
  a 
  whole. 
  

  

  Although, 
  conchologically, 
  Morula 
  appears 
  to 
  unite 
  by 
  easy 
  stages 
  

   with 
  Driipa, 
  the 
  radula 
  is 
  frapied 
  on 
  a 
  totally 
  different 
  pattern. 
  

   The 
  base, 
  instead 
  of 
  being 
  straight, 
  or 
  gently 
  curved 
  inwards, 
  curves 
  

   doubly 
  on 
  the 
  two 
  sides 
  of 
  a 
  central 
  projecting 
  hump, 
  which 
  is 
  in 
  

   fact 
  the 
  base 
  or 
  bottom 
  of 
  the 
  central 
  cusp 
  of 
  the 
  rhachidian. 
  This 
  

   cusp 
  is 
  not, 
  as 
  in 
  Thais 
  proper, 
  a 
  mere 
  projection 
  from 
  the 
  upper 
  

   edge 
  of 
  the 
  tooth, 
  but 
  is 
  very 
  thick 
  and 
  solid, 
  and 
  is 
  carried 
  right 
  

   through 
  the 
  rhachidian 
  like 
  a 
  rounded 
  pillar, 
  producing 
  the 
  hump- 
  

   like 
  excrescence 
  in 
  the 
  centre 
  of 
  the 
  base 
  itself. 
  This 
  alone 
  would 
  

   constitute 
  a 
  wide 
  difference 
  of 
  structure. 
  Further, 
  the 
  two 
  side 
  

   cusps 
  in 
  Iforula 
  are 
  never 
  denticulate, 
  either 
  on 
  the 
  inside 
  or 
  on 
  

   the 
  out, 
  the 
  knob 
  is 
  usually 
  flattened, 
  and 
  there 
  are 
  no 
  small 
  

   denticles 
  between 
  it 
  and 
  the 
  side 
  cusps, 
  nothing 
  but 
  a 
  broad 
  bare 
  

  

  