﻿COOKR 
  : 
  THE 
  RADULA 
  IN 
  THAIS, 
  ETC. 
  105 
  

  

  space. 
  Betwfjen 
  the 
  central 
  and 
  side 
  cusps 
  on 
  each 
  side 
  there 
  is 
  

   always 
  one 
  mrJi 
  independent 
  denticle, 
  rather 
  nearer 
  to 
  tlie 
  side 
  

   cusps 
  tha 
  the 
  cen^^ral, 
  deeply 
  rooted 
  in 
  the 
  mass 
  of 
  the 
  tooth, 
  

  

  and 
  on 
  a 
  lo 
  r 
  plane 
  than 
  the 
  side 
  cusps, 
  llarely 
  a 
  tiny 
  subsidiary 
  

   denticle 
  may 
  be 
  detected 
  inside 
  this 
  denticle. 
  All 
  the 
  cusps 
  are 
  

   narrow 
  and 
  sharp, 
  and 
  the 
  side 
  cusps 
  sometimes 
  incline 
  towards 
  the 
  

   central, 
  sometimes 
  curve 
  away 
  from 
  it. 
  The 
  central 
  cusp, 
  instead 
  

   of 
  being 
  more 
  or 
  less 
  on 
  a 
  level 
  with 
  the 
  other 
  cusps 
  on 
  the 
  upper 
  

   edge 
  of 
  the 
  tooth, 
  as 
  in 
  Thais, 
  is 
  always 
  depressed 
  in 
  a 
  sort 
  of 
  

   shallow 
  pit 
  or 
  curve, 
  so 
  that 
  the 
  front 
  edge 
  seems 
  to 
  slope 
  towards 
  

   the 
  central 
  cusp 
  and 
  rise 
  in 
  the 
  direction 
  of 
  the 
  knobs. 
  The 
  

   extreme 
  thickness 
  of 
  the 
  central 
  cusp 
  throws 
  it, 
  in 
  the 
  microscope, 
  

   into 
  quite 
  a 
  different 
  plane 
  of 
  vision 
  to 
  that 
  of 
  the 
  other 
  cusps. 
  The 
  

   whole 
  tooth 
  is 
  very 
  wide 
  in 
  proportion 
  to 
  its 
  depth 
  ; 
  the 
  blade 
  of 
  the 
  

   laterals 
  is 
  narrow, 
  their 
  base 
  often 
  prolonged. 
  (Fig. 
  32.) 
  

  

  None 
  of 
  the 
  different 
  species 
  of 
  Morula 
  show 
  any 
  variation 
  from 
  

   this 
  type 
  of 
  rhachidian, 
  the 
  sole 
  difference 
  lying 
  in 
  the 
  relative 
  size 
  

   and 
  sharpness 
  of 
  the 
  cusps, 
  or 
  the 
  angle 
  of 
  their 
  inclination. 
  Thus, 
  

   while 
  the 
  rhachidian 
  of 
  Morula 
  is 
  an 
  excellent 
  indication 
  of 
  the 
  

   genus, 
  it 
  is 
  by 
  no 
  means 
  so 
  distinctive, 
  as 
  regards 
  the 
  species, 
  as 
  the 
  

   radula 
  of 
  Thais. 
  

  

  No 
  author 
  seems 
  to 
  have 
  drawn 
  attention 
  to 
  this 
  very 
  characteristic 
  

   form 
  of 
  radula. 
  

  

  The 
  following 
  species 
  have 
  been 
  examined 
  : 
  — 
  

   anaxares, 
  Duel.: 
  Umkomaas. 
  Central 
  cusp 
  rather 
  short; 
  side 
  

   cusps 
  blunt 
  and 
  broad 
  ; 
  a 
  small 
  bluntish 
  denticle 
  inside 
  the 
  side 
  cusp 
  

   r 
  id 
  close 
  to 
  it. 
  

  

  cancellata, 
  Quoy 
  : 
  Umkomaas. 
  Central 
  cusp 
  very 
  thick 
  ; 
  side 
  

   c.isps 
  inclined 
  inwards 
  ; 
  a 
  tiny 
  denticle 
  inside, 
  very 
  low 
  down. 
  

  

  chaidea. 
  Duel. 
  : 
  Tonga. 
  Central 
  cusp 
  very 
  long 
  and 
  sharp 
  ; 
  side 
  

   cusps 
  small, 
  pointed; 
  one 
  small 
  denticle, 
  pointing 
  inwards, 
  low 
  

   down 
  on 
  interior 
  side 
  (this 
  denticle 
  is 
  not 
  always 
  present; 
  its 
  

   distance 
  from 
  the 
  side 
  cusp, 
  and 
  therefore 
  from 
  the 
  central, 
  differs 
  

   in 
  dit'erent 
  specimens; 
  in 
  another 
  Tonga 
  specimen 
  it 
  is 
  quite 
  close 
  

   to 
  the' 
  cusp). 
  

  

  chrysostoma, 
  Desh. 
  : 
  Karaclii. 
  Central 
  cusp 
  normal; 
  side 
  cusps 
  

   two-thirds 
  length 
  of 
  central, 
  mucli 
  broader, 
  point 
  inclining 
  outwards; 
  

   inside 
  denticle 
  long, 
  narrow, 
  and 
  sliarp, 
  inclining 
  towards 
  central 
  cusp. 
  

   concatenata, 
  Lam.: 
  Karachi. 
  Central 
  cusp 
  not 
  very 
  long; 
  side 
  

   cusp 
  broad 
  and 
  humpy, 
  tlie 
  point 
  turning 
  markedly 
  outwards; 
  

   a 
  large 
  prominent 
  denticle 
  close 
  to 
  the 
  base 
  of 
  the 
  side 
  cusp, 
  but 
  

   quite 
  separate 
  from 
  it, 
  and 
  set 
  nearer 
  the 
  upper 
  margin 
  of 
  the 
  tooth 
  

   than 
  is 
  usual 
  in 
  this 
  group. 
  

  

  On 
  the 
  other 
  hand, 
  a 
  specimen 
  from 
  Umkomaas 
  (H. 
  C. 
  Burnup) 
  

   is 
  of 
  the 
  Thais 
  type; 
  central 
  cusp 
  long, 
  thick, 
  narrow, 
  sharply 
  

   pointed, 
  side 
  cusps 
  very 
  small, 
  bluntly 
  triangular, 
  with 
  a 
  blunt 
  

   hump 
  rather 
  than 
  a 
  denticle, 
  at 
  the 
  inner 
  base 
  ; 
  outside 
  are 
  5-6 
  

   small 
  denticles, 
  two 
  of 
  which 
  climb; 
  knob 
  prominent; 
  base 
  that 
  of 
  

   Thais. 
  I 
  think 
  this 
  specimen 
  is 
  misidentified, 
  and 
  that 
  the 
  species 
  

   loe 
  * 
  ^ong 
  to 
  the 
  Morula 
  group. 
  

  

  m.— 
  APRIL, 
  1919. 
  8 
  

  

  