﻿50 
  W. 
  B. 
  HANDLES. 
  

  

  elongated 
  bodies, 
  whose 
  protoplasm 
  is 
  finely 
  granular, 
  the 
  

   granules 
  being 
  arranged 
  in 
  longitudinal 
  strise 
  ; 
  each 
  cell 
  

   encloses 
  a 
  large 
  oval 
  nucleus. 
  

  

  The 
  formative 
  cells 
  rest 
  upon 
  a 
  clear, 
  thin, 
  structureless 
  

   basement 
  membrane 
  {h. 
  m.), 
  which 
  is 
  in 
  turn 
  succeeded 
  by 
  a 
  

   layer 
  of 
  muscle-fibres 
  [m. 
  f.) 
  with 
  elongated 
  nuclei. 
  

  

  In 
  many 
  of 
  the 
  exotic 
  Trochidte 
  (e.g. 
  T. 
  niloticus, 
  etc.) 
  

   jaws 
  are 
  entirely 
  absent. 
  

  

  Closely 
  attached 
  to 
  the 
  bod\'-wall 
  by 
  radiating 
  mnsclc- 
  

   fibres 
  is 
  the 
  buccal 
  mass 
  (figs. 
  39, 
  40, 
  44) 
  ; 
  this 
  is 
  a 
  very 
  

   muscular 
  structure, 
  and 
  is 
  supported 
  by 
  the 
  large 
  odontophore 
  

   {od.), 
  consisting 
  of 
  two 
  pairs 
  of 
  odontophoral 
  cartilages; 
  the 
  

   larger 
  and 
  anterior 
  pair 
  serve 
  mainly 
  for 
  the 
  support 
  of 
  the 
  

   radula, 
  while 
  the 
  smaller 
  basal 
  and 
  posterior 
  pair 
  present 
  

   concave 
  surfaces 
  upon 
  which 
  the 
  anterior 
  cartilages 
  articulate, 
  

   and 
  also 
  serve 
  as 
  fixed 
  points 
  for 
  the 
  attachment 
  of 
  the 
  

   majority 
  of 
  the 
  protractor 
  and 
  retractor 
  muscles 
  of 
  the 
  

   odontophore. 
  

  

  The 
  radula 
  is 
  extremely 
  long, 
  and 
  is 
  ensheathed 
  in 
  a 
  

   radula-sac 
  (r. 
  s.), 
  Avhich, 
  after 
  emerging 
  from 
  between 
  the 
  

   anterior 
  pair 
  of 
  odontophoral 
  cartilages, 
  becomes 
  involved 
  in 
  

   the 
  general 
  torsion 
  of 
  the 
  body, 
  and, 
  though 
  situated 
  ventral 
  

   to 
  the 
  crop 
  anteriorly, 
  is 
  twisted 
  over 
  the 
  right 
  side, 
  so 
  that 
  

   the 
  posterior 
  portion 
  eventually 
  comes 
  to 
  lie 
  on 
  the 
  dorsal 
  

   surface 
  of 
  the 
  crop. 
  

  

  The 
  terminal 
  portion 
  of 
  the 
  radula-sac 
  is 
  bifid 
  in 
  T. 
  

   lineatus 
  (fig. 
  40, 
  r. 
  s.), 
  T. 
  magus 
  (fig. 
  39, 
  r. 
  s.), 
  and 
  all 
  

   other 
  species 
  belonging 
  to 
  the 
  sub-genus 
  Gibbula, 
  In 
  

   T. 
  granulatus 
  and 
  T. 
  zizyphinus 
  there 
  is 
  no 
  trace 
  what- 
  

   ever 
  of 
  this 
  bifurcation. 
  

  

  The 
  radula 
  of 
  Trochus 
  is 
  typically 
  rhipidoglossate. 
  

   Troschel 
  (42) 
  has 
  figured 
  and 
  described 
  the 
  radulee 
  of 
  

   numerous 
  species 
  of 
  the 
  Troehida3. 
  

  

  Amongst 
  the 
  species 
  enumerated 
  in 
  this 
  paper 
  very 
  little 
  

   difference 
  in 
  radula 
  structure 
  occurs. 
  We 
  can, 
  however, 
  

   distinguish 
  between 
  two 
  fairly 
  distinct 
  types, 
  represented 
  by 
  

   T. 
  granulatus 
  and 
  T. 
  zizyphinus 
  on 
  the 
  one 
  hand 
  and 
  

  

  