﻿36 
  W. 
  B. 
  RANDLBS. 
  

  

  two 
  chai'acters 
  are 
  common, 
  however, 
  to 
  several 
  species 
  of 
  

   Gibbula 
  and 
  the 
  typical 
  section 
  Zizyphiuus, 
  which 
  last 
  

   has 
  a 
  pyramidal 
  spire. 
  It 
  is 
  also 
  not 
  generally 
  known, 
  but 
  

   not 
  less 
  the 
  fact, 
  that 
  young 
  shells 
  of 
  T. 
  lineatus 
  (the 
  type 
  

   of 
  Trocho 
  cochlea) 
  are 
  always 
  deeply 
  umbilicate.^^ 
  

  

  We 
  see, 
  then, 
  that 
  the 
  conchological 
  differences 
  between 
  the 
  

   two 
  sub-genera 
  are 
  very 
  meagre 
  and 
  valueless 
  for 
  diagnosis; 
  

   and 
  when 
  we 
  come 
  to 
  compare 
  their 
  anatomical 
  structure, 
  we 
  

   find 
  they 
  are 
  so 
  nearly 
  identical 
  that 
  it 
  seems 
  quite 
  unneces- 
  

   sary 
  for 
  the 
  separate 
  sub-genus 
  to 
  be 
  retained. 
  

  

  The 
  species 
  6 
  — 
  9, 
  however, 
  fall 
  into 
  a 
  group 
  quite 
  distinct 
  

   from 
  that 
  of 
  Gibbula, 
  and 
  exhibit 
  anatomical 
  differences 
  

   that 
  warrant 
  their 
  separation 
  into 
  a 
  sub-genus, 
  viz. 
  C 
  al- 
  

   lies 
  torn 
  a. 
  Here, 
  however, 
  although 
  T. 
  zizyphinus 
  and 
  

   T. 
  granulatus 
  are 
  very 
  different 
  in 
  many 
  respects 
  from 
  any 
  

   species 
  of 
  Gibbula, 
  some 
  of 
  the 
  smaller 
  species 
  of 
  Callio- 
  

   stoma, 
  viz. 
  T. 
  striatus, 
  and 
  T. 
  ex 
  asp 
  e 
  rat 
  us, 
  present 
  

   points 
  of 
  startling 
  similarity 
  in 
  the 
  radula 
  and 
  some 
  external 
  

   features 
  to 
  T. 
  magus 
  and 
  other 
  species 
  of 
  G 
  ibbula. 
  They, 
  

   however, 
  in 
  possessing 
  pyramidal 
  shells, 
  and 
  iu 
  tlie 
  presence 
  

   of 
  an 
  accessory 
  structure 
  in 
  connection 
  with 
  the 
  female 
  genital 
  

   organs 
  (a 
  structure 
  common 
  to 
  all 
  the 
  British 
  species 
  of 
  Cal- 
  

   liostoma 
  which 
  I 
  have 
  examined), 
  undoubtedly 
  belong 
  to 
  

   this 
  latter 
  sub-genus. 
  

  

  External 
  Characters. 
  — 
  The 
  head 
  is 
  moderately 
  large, 
  

   and 
  is 
  bent 
  downwards 
  into 
  a 
  cylindrical 
  snout, 
  on 
  the 
  under- 
  

   surface 
  of 
  which 
  is 
  situated 
  the 
  mouth. 
  There 
  are 
  pi-esent 
  

   on 
  either 
  side 
  of 
  the 
  head 
  three 
  appendages, 
  the 
  outermost 
  of 
  

   these, 
  the 
  ocular 
  peduncles 
  (figs. 
  5, 
  6, 
  7, 
  oc. 
  p.) 
  are 
  short, 
  

   laterally 
  flattened 
  structures, 
  presenting 
  in 
  cross-section 
  a 
  

   somewhat 
  oval 
  contour. 
  Near 
  the 
  apices 
  of 
  these 
  the 
  eyes 
  

   are 
  situated. 
  Internal 
  to 
  the 
  ocular 
  ])eduncles 
  are 
  placed 
  

   the 
  cephalic 
  tentacles, 
  highly 
  muscular 
  organs, 
  capable 
  of 
  

   great 
  extension 
  and 
  covered 
  externally 
  with 
  fine 
  cilia 
  (fig. 
  7, 
  

   c. 
  t.) 
  

  

  An 
  interesting 
  condition 
  is 
  seen 
  in 
  the 
  larval 
  forms 
  of 
  

   Trochus 
  (vide 
  Robert, 
  38, 
  fig. 
  508, 
  k) 
  — 
  the 
  cephalic 
  tentacles 
  

  

  