﻿PIISBKY 
  : 
  ON 
  LITHASI0PSI8, 
  N.GEN. 
  

  

  49 
  

  

  The 
  aperture 
  usually 
  exceeds 
  half 
  the 
  total 
  length, 
  is 
  ovate 
  or 
  

   piriform 
  in 
  shape, 
  and 
  white 
  within. 
  The 
  outer 
  lip 
  is 
  thin 
  and 
  

   sharp, 
  arcuate 
  or 
  straightened 
  ; 
  basal 
  margin 
  arcuate. 
  Columella 
  

   concave. 
  Columellar 
  and 
  parietal 
  margins 
  are 
  covered 
  with 
  a 
  very 
  

   heavy 
  white 
  callus, 
  which 
  is 
  thickened 
  into 
  a 
  low 
  nodule 
  or 
  pad 
  

   near 
  the 
  posterior 
  angle 
  of 
  the 
  aperture. 
  

  

  Length 
  13"8, 
  diam. 
  7"1 
  mm. 
  

  

  „ 
  13-5 
  ,, 
  7-9 
  „ 
  

  

  ,, 
  12-3 
  ,, 
  6-8 
  ,, 
  

   11-9 
  7 
  

  

  The 
  operculum 
  is 
  rounded-ovate, 
  smaller 
  and 
  especially 
  shorter 
  

   than 
  the 
  aperture. 
  

  

  Sab. 
  — 
  Coy 
  E.iver, 
  a 
  tributary 
  of 
  the 
  Panuco, 
  State 
  of 
  San 
  Luis 
  

   Potosi, 
  Mexico. 
  

  

  Fig. 
  3. 
  Lithasiopsis 
  Sinkleyi. 
  Two 
  adult 
  and 
  one 
  young 
  example. 
  

  

  This 
  species 
  has 
  much 
  in 
  common 
  with 
  Lithasia 
  olovata 
  (Say). 
  

   The 
  general 
  shape, 
  colour, 
  and 
  columellar 
  callus 
  are 
  similar; 
  but 
  the 
  

   operculum 
  differs 
  widely. 
  The 
  shape 
  of 
  shell 
  and 
  aperture 
  are 
  

   remarkably 
  variable, 
  but 
  even 
  in 
  the 
  more 
  ovate 
  and 
  lengthened 
  

   examples, 
  the 
  last 
  whorl 
  is 
  decidedly 
  shouldered, 
  unlike 
  the 
  following 
  

   species. 
  As 
  in 
  many 
  Pleuroceratidse, 
  smooth 
  and 
  spirally 
  striate 
  

   forms 
  coexist 
  in 
  the 
  same 
  colonies. 
  

  

  Lithasiopsis 
  Mexicanus, 
  n.sp. 
  Fig. 
  4. 
  

   The 
  shell 
  is 
  ovate-turrite, 
  with 
  straightly 
  conic 
  spire 
  ; 
  solid, 
  

   lustreless, 
  variously 
  coloured 
  ; 
  olive-yellow, 
  uniform 
  or 
  with 
  a 
  blackish 
  

   olive 
  median 
  band, 
  or 
  purplish 
  black, 
  paler 
  below 
  the 
  suture. 
  The 
  

   first 
  2^ 
  whorls 
  are 
  smooth 
  and 
  convex, 
  the 
  following 
  whorls 
  flattened, 
  

   at 
  first 
  smooth, 
  but 
  about 
  the 
  fifth 
  whorl 
  a 
  few 
  spiral 
  threads 
  

   appear. 
  These 
  threads 
  continue 
  to 
  the 
  penultimate 
  whorl, 
  where 
  

   they 
  decrease, 
  leaving 
  the 
  last 
  whorl 
  smooth 
  ; 
  or 
  they 
  may 
  persist 
  

   as 
  very 
  weak 
  angles 
  on 
  the 
  last 
  whorl. 
  Whorls 
  9 
  or 
  10 
  in 
  complete 
  

   examples, 
  but 
  the 
  earliest 
  ones 
  are 
  generally 
  worn 
  or 
  eroded 
  in 
  

   adult 
  individuals. 
  Last 
  whorl 
  convex. 
  Aperture 
  ovate, 
  livid 
  white 
  

  

  VOL. 
  IX. 
  — 
  MARCH, 
  1910. 
  4 
  

  

  