﻿COOKE 
  : 
  ON 
  GENERIC 
  POSITION 
  OF 
  NOnTHIA. 
  237 
  

  

  significant 
  feature 
  of 
  an 
  Alectrion 
  radula, 
  the 
  numerous 
  small 
  

   denticles 
  of 
  the 
  rhachidian, 
  smaller 
  at 
  the 
  sides 
  than 
  in 
  the 
  middle, 
  

   is 
  absent 
  altogether. 
  

  

  In 
  its 
  place 
  we 
  find 
  on 
  the 
  rhachidian 
  tooth 
  three 
  large 
  strong 
  

   denticles, 
  the 
  central 
  denticle 
  rather 
  the 
  largest, 
  base 
  narrow, 
  sides 
  

   squarish, 
  strongly 
  winged. 
  The 
  laterals 
  are 
  bicuspid, 
  simple, 
  exterior 
  

   cusp 
  longer 
  and 
  thinner 
  than 
  the 
  interior 
  cusp, 
  which 
  is 
  strong 
  and 
  

   remarkably 
  broad. 
  

  

  This 
  combination 
  of 
  a 
  bicuspid 
  lateral, 
  having 
  the 
  inner 
  cusp 
  

   the 
  broader 
  and 
  stouter, 
  with 
  a 
  tricuspid 
  rhachidian, 
  having 
  cusps 
  

   nearly 
  equal 
  in 
  size, 
  set 
  on 
  a 
  narrow 
  base 
  with 
  sides 
  prolonged 
  

   into 
  wings, 
  is 
  characteristic 
  of 
  a 
  section 
  of 
  the 
  genus 
  Fhos, 
  to 
  which 
  

   Northia 
  stands 
  undoubtedly 
  in 
  very 
  close 
  relationship. 
  Compare 
  

   tlie 
  figures 
  of 
  the 
  radulse 
  of 
  Phoi 
  senticosum, 
  Lam., 
  and 
  I'hos 
  

   cmicellatiim, 
  Quoy, 
  given 
  in 
  Troschel, 
  Das 
  Gehiss 
  der 
  Schneclcen, 
  

   vol. 
  ii, 
  pi. 
  viii, 
  f. 
  1, 
  2. 
  A 
  very 
  sharp 
  subdivision 
  of 
  the 
  species 
  

   hitherto 
  united 
  under 
  Fhos 
  is 
  indicated 
  by 
  an 
  examination 
  of 
  the 
  

   rhachidian 
  tooth, 
  for, 
  besides 
  senticosum, 
  Lam., 
  and 
  cancellatum^ 
  Quoy, 
  

   roneatiim, 
  Hinds, 
  and 
  varians, 
  Sowb., 
  possess 
  the 
  tricuspid 
  rhachidian 
  

   (and 
  so 
  also 
  does 
  (asmanicum,^ 
  Ten. 
  -Woods), 
  while 
  gaudens, 
  Hinds, 
  

   nodicostatum, 
  A. 
  Ad., 
  and 
  pallidum, 
  Hinds, 
  have 
  a 
  rhachidian 
  with 
  

   more 
  numerous 
  denticles, 
  of 
  an 
  Alectriun 
  type. 
  

  

  So 
  far 
  as 
  the 
  radula 
  is 
  concerned, 
  Northia 
  and 
  Phos 
  stand 
  in 
  

   somewhat 
  close 
  relationship 
  to 
  the 
  Conwiellce 
  of 
  New 
  Zealand 
  and 
  

   Australia, 
  and, 
  to 
  a 
  less 
  degree, 
  to 
  certain 
  Australo-Neozealanian 
  

   species 
  of 
  Euthria. 
  All 
  these 
  four 
  genera 
  are 
  much 
  more 
  closely 
  

   related 
  to 
  Buccimim 
  than 
  they 
  are 
  to 
  Alectrio7i, 
  and 
  should 
  be 
  

   grouped 
  accordingly. 
  

  

  The 
  shape 
  of 
  the 
  operculum 
  of 
  Northia 
  confirms 
  the 
  view 
  that 
  the 
  

   genus 
  is 
  related 
  to 
  the 
  Photidaj 
  and 
  not 
  to 
  the 
  Alectrionida). 
  As 
  

   given 
  in 
  the 
  figure 
  of 
  H. 
  & 
  A. 
  Adams' 
  "Genera", 
  pi. 
  xii, 
  \a, 
  \h, 
  

   it 
  is 
  ratlier 
  thick, 
  long, 
  and 
  narrow, 
  narrowing 
  towards 
  the 
  nucleus, 
  

   which 
  is 
  terminal 
  and 
  slightly 
  hooked. 
  That 
  of 
  Phos 
  is 
  similar, 
  

   though 
  not 
  quite 
  so 
  thick, 
  with 
  a 
  sub-terminal 
  nucleus, 
  Adams' 
  

   figure 
  of 
  Northia 
  is 
  not 
  quite 
  correct 
  ; 
  specimens 
  do 
  not 
  show 
  a 
  marked 
  

   notch 
  close 
  to 
  the 
  terminal 
  nucleus. 
  

  

  It 
  should 
  be 
  noted 
  that 
  the 
  two 
  other 
  species 
  (albopunctata, 
  

   Ad. 
  & 
  Reeve, 
  and 
  rissoides, 
  Reeve) 
  classified 
  as 
  Northia 
  by 
  the 
  

   Adams 
  and 
  by 
  Tryon, 
  have 
  no 
  claim 
  to 
  that 
  position. 
  Still 
  less 
  is 
  

   Pleurotoma 
  (^Daphnella) 
  boholensis, 
  lleeve, 
  a 
  Northia, 
  as 
  suggested 
  by 
  

   Couturier 
  (Journ. 
  de 
  Conch., 
  tom. 
  iv, 
  1907, 
  p. 
  130). 
  

  

  ^ 
  J. 
  C. 
  Verco, 
  Trans. 
  Roy. 
  Soc. 
  S. 
  Austr., 
  vol. 
  xx, 
  1896, 
  p. 
  228, 
  pi. 
  viii, 
  f. 
  6. 
  

  

  