﻿ANATOMY 
  AND 
  AFFINITIES 
  OF 
  THE 
  TUOCHlDiE. 
  51 
  

  

  T. 
  magus 
  and 
  tlie 
  remaining 
  species 
  on 
  the 
  other. 
  In 
  the 
  

   former 
  (figs. 
  20, 
  21) 
  the 
  radula 
  is 
  characterised 
  by 
  the 
  

   extremely 
  large 
  size 
  of 
  the 
  first 
  or 
  admedian 
  marginal 
  toothy 
  

   also 
  by 
  the 
  serrated 
  edges 
  of 
  the 
  cusps 
  of 
  both 
  the 
  central 
  

   and 
  lateral 
  teeth. 
  In 
  the 
  latter 
  the 
  cusps 
  of 
  the 
  central 
  and 
  

   lateral 
  teeth 
  are 
  devoid 
  of 
  serrations^ 
  but 
  the 
  lateral 
  teeth 
  are 
  

   notched 
  on 
  their 
  distal 
  margins^ 
  and 
  the 
  central 
  tooth 
  has 
  

   notches 
  on 
  both 
  sides 
  of 
  the 
  basal 
  portion 
  of 
  the 
  cusp 
  (figs. 
  

   15j 
  18, 
  19, 
  28, 
  29). 
  The 
  first 
  marginal 
  tooth 
  of 
  these 
  species 
  

   is 
  also 
  of 
  considerable 
  size, 
  but 
  not 
  so 
  large 
  relatively 
  as 
  in 
  

   T. 
  granulatus 
  or 
  T. 
  zizyphinus. 
  In 
  T. 
  lineatus 
  (fig. 
  19), 
  

   on 
  the 
  contrary, 
  the 
  first 
  marginal 
  tooth 
  differs 
  in 
  no 
  way 
  

   from 
  the 
  succeeding 
  ones. 
  

  

  In 
  each 
  trausverse 
  row 
  of 
  teeth 
  of 
  the 
  radula 
  of 
  Trochus 
  

   the 
  following 
  clearly 
  defined 
  regions 
  can 
  be 
  distinguished. 
  

   An 
  unpaired 
  median 
  or 
  rachidian 
  tooth, 
  bordered 
  on 
  either 
  

   side 
  by 
  five 
  lateral 
  teeth, 
  succeeding 
  which 
  is 
  an 
  indefinite 
  

   number 
  of 
  marginal 
  teeth 
  or 
  uncini. 
  We 
  can 
  represent 
  

   the 
  dentition 
  of 
  the 
  radula 
  by 
  a 
  formula 
  as 
  follows 
  : 
  

  

  c» 
  5 
  1 
  5 
  oc 
  

  

  The 
  marginal 
  teeth 
  vary 
  considerably 
  iu 
  shape 
  and 
  size, 
  

   those 
  nearer 
  the 
  central 
  tooth 
  being 
  stouter 
  and 
  shorter 
  than 
  

   those 
  more 
  remote. 
  The 
  majority 
  of 
  the 
  marginal 
  teeth 
  or 
  

   uncini 
  are 
  hooked 
  (figs. 
  16, 
  17, 
  22 
  — 
  24). 
  The 
  teeth 
  situated 
  

   some 
  distance 
  from 
  the 
  centre 
  become 
  slender 
  and 
  elongate 
  

   (figs. 
  21', 
  25). 
  In 
  T. 
  zizyphinus 
  and 
  T. 
  granulatus 
  

   these 
  distal 
  teeth 
  are 
  characterised 
  by 
  the 
  deep 
  serrations 
  

   on 
  the 
  margins. 
  In 
  teeth 
  still 
  more 
  remote 
  these 
  serra- 
  

   tions 
  (fig. 
  20) 
  become 
  still 
  deeper, 
  and 
  give 
  a 
  brush-like 
  

   appearance 
  to 
  the 
  teeth, 
  though 
  they 
  cannot 
  bo 
  compared 
  

   to 
  the 
  brush-teeth 
  of 
  Pleurotomaria 
  (45, 
  p. 
  250, 
  figs. 
  

   4G— 
  52). 
  

  

  At 
  the 
  extreme 
  distal 
  end 
  of 
  the 
  marginal 
  teeth 
  some 
  nine 
  

   or 
  ten 
  specialised 
  teeth 
  are 
  situated. 
  These 
  are 
  flattened, 
  and 
  

   present 
  neither 
  serration* 
  nor 
  notches 
  on 
  the 
  margins. 
  They 
  

  

  