﻿92 
  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETT. 
  

  

  feature 
  lias 
  been 
  noted 
  in 
  a 
  considerable 
  number 
  of 
  the 
  species 
  

   examined. 
  

  

  Thus 
  one 
  finds 
  it 
  possible 
  to 
  arrange 
  the 
  greater 
  number 
  of 
  the 
  

   species 
  of 
  Thais 
  in 
  a 
  continuous 
  series, 
  starting 
  from 
  a 
  rhachidian 
  

   tooth 
  of 
  extreme 
  simplicity, 
  sucli 
  as 
  that 
  of 
  gemmidata, 
  in 
  which 
  

   there 
  are 
  no 
  denticles 
  whatever 
  and 
  scarcely 
  any 
  knob, 
  nothing 
  but 
  

   three 
  bare 
  cusps, 
  through 
  a 
  series 
  in 
  -which 
  the 
  development 
  of 
  

   denticles 
  is 
  observed, 
  first 
  on 
  the 
  inside 
  of 
  the 
  side 
  cusps 
  and 
  

   afterwards 
  between 
  tlie 
  side 
  cusps 
  and 
  the 
  knob, 
  and 
  then 
  climbing 
  

   up 
  the 
  exterior 
  side 
  of 
  the 
  cusps 
  tliemselves, 
  till 
  the 
  series 
  reaches 
  

   its 
  extreme 
  of 
  complexity 
  in 
  u 
  rhachidian 
  tooth 
  such 
  as 
  that 
  of 
  

   hcemastoma. 
  

  

  No 
  satisfactory' 
  break 
  in 
  the 
  series 
  occurs 
  ■which 
  would 
  induce 
  one 
  

   to 
  subdivide 
  it 
  up 
  into 
  groups. 
  On 
  the 
  contrary, 
  the 
  passage 
  from 
  

   a 
  simple 
  to 
  a 
  complex 
  form 
  is 
  gradual, 
  and 
  one 
  can 
  discern 
  no 
  

   adequate 
  reason 
  for 
  drawing 
  a 
  line 
  of 
  separation 
  in 
  one 
  place 
  more 
  

   than 
  in 
  another. 
  If 
  the 
  species 
  in 
  the 
  so-called 
  group 
  Thalessa, 
  as 
  

   listed 
  by 
  Tryon, 
  were 
  picked 
  out 
  of 
  a 
  long 
  series 
  so 
  arranged, 
  it 
  

   would 
  be 
  found 
  that 
  some 
  of 
  them, 
  e.g. 
  gemtmilata, 
  hippocastanea, 
  

   pica, 
  were 
  at 
  one 
  end, 
  others, 
  e.g. 
  armigera, 
  htifo, 
  were 
  at 
  the 
  other, 
  

   while 
  others, 
  e.g. 
  melones, 
  oieritoidea, 
  would 
  be 
  in 
  the 
  middle. 
  

  

  The 
  lateral 
  teeth 
  throughout 
  the 
  whole 
  group 
  are 
  very 
  similar, 
  

   and 
  ofi^er 
  little 
  help 
  in 
  determining 
  the 
  relationship 
  of 
  the 
  various 
  

   forms. 
  

  

  eegrota, 
  Reeve 
  : 
  Safety 
  Eay, 
  AV. 
  Austr. 
  Central 
  cusp 
  broad, 
  not 
  

   very 
  long, 
  side 
  cusps 
  long; 
  faintly 
  denticled 
  very 
  low 
  down 
  on 
  

   inside, 
  almost 
  beliind 
  the 
  cusps 
  ; 
  four 
  or 
  five 
  small 
  but 
  well-marked 
  

   denticles, 
  one 
  at 
  least 
  climbing. 
  

  

  armigera, 
  Chem. 
  : 
  S. 
  Pacific. 
  Central 
  cusp 
  rather 
  broad, 
  but 
  

   narrowing 
  sharply 
  towards 
  the 
  tip 
  ; 
  side 
  cusps 
  exactly 
  the 
  same 
  

   shape, 
  and 
  about 
  two-thirds 
  the 
  length, 
  set 
  rather 
  distant 
  from 
  the 
  

   central, 
  conspicuously 
  bluntly 
  denticled 
  low 
  down 
  on 
  inside, 
  denticle 
  

   almost 
  clear 
  of 
  the 
  cusp, 
  well 
  rooted 
  in 
  the 
  mass 
  of 
  the 
  tooth 
  ; 
  

   outside, 
  six 
  to 
  seven 
  small 
  sharp 
  denticles, 
  three 
  of 
  which 
  climb, 
  

   giving 
  the 
  outer 
  edge 
  of 
  the 
  cusp 
  quite 
  a 
  serrated 
  appearance 
  ; 
  knob 
  

   conspicuous; 
  base 
  slightly 
  broadly 
  waved 
  in 
  the 
  centre. 
  (Kg. 
  10.) 
  

  

  A 
  specimen 
  from 
  Mauritius 
  has 
  only 
  five 
  denticles, 
  not 
  climbing 
  

   so 
  high. 
  

  

  bicostalis, 
  Lam. 
  : 
  Panama. 
  Central 
  cusp 
  long, 
  rather 
  narrow, 
  

   sharply 
  pointed, 
  a 
  "guard" 
  plainly 
  visible 
  towards 
  the 
  lower 
  part 
  

   of 
  the 
  sides; 
  side 
  cusps 
  much 
  broader, 
  sharply 
  triangular, 
  

   prominently 
  denticled 
  very 
  high 
  up 
  on 
  the 
  inside, 
  higher 
  than 
  in 
  

   any 
  other 
  species 
  of 
  Thais, 
  denticle 
  not 
  deeply 
  cut 
  ; 
  outside, 
  three 
  to 
  

   five 
  well-marked 
  sharp 
  denticles, 
  one 
  to 
  two 
  climbing 
  ; 
  knob 
  

   prominent, 
  narrow 
  ; 
  base 
  straight. 
  (Fig. 
  13.) 
  

  

  In 
  another 
  specimen 
  (same 
  locality) 
  the 
  inner 
  denticle 
  is 
  quite 
  close 
  

   to 
  the 
  tip 
  of 
  the 
  cusp. 
  Troschel's 
  figure 
  is 
  very 
  incorrect. 
  

  

  lituhercularis, 
  Lam. 
  : 
  Karachi. 
  Central 
  cusp 
  prominent, 
  broad 
  at 
  

   base, 
  narrowing 
  rapidly 
  towards 
  the 
  point, 
  side 
  cusps 
  veiy 
  broad, 
  

   'conspicuously 
  denticled 
  low 
  down 
  on 
  the 
  inside; 
  outside, 
  two 
  very 
  

  

  