﻿POMATIOPSIS 
  LAPIDARIA 
  — 
  ABBOTT 
  61 
  

  

  Previously 
  unrecorded 
  in 
  either 
  Oncoinelania 
  or 
  Pomatiopsis 
  is 
  a 
  

   peculiar 
  modification 
  of 
  the 
  digestive 
  system. 
  Within 
  the 
  stomach 
  

   is 
  a 
  thin, 
  clear 
  lining 
  (of 
  chitin?), 
  which 
  is 
  crudely 
  shaped 
  into 
  a 
  

   corkscrew 
  funnel. 
  Flaps 
  extending 
  from 
  the 
  funnel 
  line 
  the 
  walls 
  

   of 
  the 
  stomach 
  in 
  its 
  lower 
  or 
  posterior 
  half. 
  It 
  has 
  been 
  observed 
  

   in 
  living 
  and 
  feeding 
  snails 
  that 
  the 
  mixture 
  of 
  food 
  and 
  sand, 
  pass- 
  

   ing 
  from 
  the 
  esophagus 
  into 
  the 
  stomach, 
  is 
  forced 
  through 
  the 
  center 
  

   of 
  the 
  funnel, 
  thus 
  grinding 
  the 
  sand 
  particles 
  and 
  food 
  together. 
  

   The 
  contents 
  swirl 
  around 
  at 
  the 
  rate 
  of 
  about 
  50 
  revolutions 
  per 
  

   minute. 
  Within 
  the 
  lower 
  part 
  of 
  the 
  stomach 
  and 
  partially 
  sheathed 
  

   by 
  the 
  auxiliary 
  flaps 
  of 
  this 
  gastric 
  sheath 
  is 
  a 
  jellylike, 
  elliptical, 
  

   crystalline 
  style, 
  which 
  in 
  other 
  mollusks 
  is 
  known 
  to 
  produce 
  digestive 
  

   enzymes. 
  In 
  Oncomelania 
  the 
  gastric 
  sheath 
  and 
  crystalline 
  style 
  are 
  

   more 
  highly 
  developed 
  than 
  in 
  Pomatiopsis. 
  O. 
  quadrasi 
  young 
  were 
  

   observed 
  to 
  pass 
  food 
  through 
  the 
  entire 
  alimentary 
  system 
  in 
  four 
  

   minutes. 
  While 
  the 
  swirling 
  motion 
  through 
  the 
  stomach 
  continued 
  

   as 
  long 
  as 
  there 
  was 
  food 
  present, 
  the 
  radula 
  scraped 
  periodicall}'^ 
  at 
  

   the 
  rate 
  of 
  53 
  revolutions 
  per 
  minute. 
  (See 
  pi. 
  3, 
  fig. 
  5.) 
  

  

  Radula. 
  — 
  The 
  lingual 
  ribbon 
  or 
  radula 
  of 
  this 
  species 
  is 
  hardly 
  

   distinguishable 
  from 
  that 
  found 
  in 
  Oncomelania 
  hu'pensis 
  and 
  noso- 
  

   phora. 
  There 
  are 
  92 
  to 
  95 
  rows 
  of 
  teeth 
  on 
  the 
  ribbon, 
  which 
  is 
  held 
  

   together 
  by 
  a 
  thin, 
  transparent 
  membrane 
  bearing 
  two 
  side 
  wings. 
  

   Each 
  row 
  consists 
  of 
  a 
  single 
  rachidian 
  (or 
  central) 
  tooth 
  flanked 
  

   closely 
  on 
  each 
  side 
  by 
  first 
  a 
  lateral, 
  then 
  an 
  inner 
  marginal 
  and 
  

   lastly 
  by 
  an 
  outer 
  marginal 
  tooth. 
  Each 
  tooth 
  bears 
  a 
  characteristic 
  

   number 
  of 
  tiny 
  denticles. 
  Radula 
  counts 
  in 
  this 
  paper 
  refer 
  to 
  the 
  

   number 
  of 
  denticles 
  on 
  each 
  tooth. 
  P. 
  lapidaria 
  from 
  Ann 
  Arbor, 
  

  

  Mich., 
  generally 
  had 
  a 
  count 
  of 
  ^^z^; 
  2-1-3; 
  7; 
  5, 
  although 
  the 
  

   lateral 
  occasionally 
  varied 
  on 
  the 
  same 
  ribbon 
  as 
  2-1-2 
  or 
  2-1-4. 
  Some 
  

   inner 
  marginals 
  had 
  only 
  6 
  denticles. 
  Specimens 
  in 
  Washington, 
  

   D. 
  C, 
  usually 
  had 
  a 
  rachidian 
  count 
  of 
  ^^z^, 
  although 
  a 
  few 
  ran 
  

  

  -^z^" 
  • 
  Many 
  specimens 
  of 
  Oncomelania 
  hupensis 
  from 
  Hung-Jao 
  

   Road, 
  Shanghai, 
  China, 
  have 
  shown 
  these 
  same 
  counts. 
  

  

  SeoGual 
  dimorphism. 
  — 
  Population 
  studies 
  on 
  this 
  species 
  have 
  shown 
  

   a 
  difference 
  in 
  the 
  mean 
  size 
  of 
  males 
  and 
  females. 
  This 
  is 
  not 
  un- 
  

   usual 
  in 
  many 
  dioecious 
  species, 
  and 
  in 
  most 
  of 
  such 
  cases 
  the 
  males 
  

   are 
  the 
  smaller. 
  The 
  size 
  difference 
  in 
  sexes 
  and 
  the 
  mean 
  size 
  of 
  

   the 
  entire 
  population 
  vary 
  from 
  colony 
  to 
  colony. 
  The 
  histogram 
  

   (fig. 
  11) 
  represents 
  the 
  distribution 
  of 
  the 
  length 
  of 
  adult 
  males 
  and 
  

   females 
  from 
  one 
  colony. 
  The 
  clear 
  columns 
  of 
  the 
  histogram, 
  de- 
  

   noting 
  males, 
  have 
  been 
  added 
  to 
  the 
  shaded 
  areas 
  denoting 
  females, 
  

   so 
  that 
  the 
  histogram 
  is 
  a 
  sum 
  of 
  both 
  sexes. 
  This 
  population 
  from 
  

   Ann 
  Arbor, 
  Mich., 
  produces 
  a 
  well-balanced, 
  normal 
  population 
  curve, 
  

   but 
  in 
  a 
  colony 
  from 
  Grand 
  Rapids, 
  Mich., 
  the 
  lengths 
  of 
  the 
  two 
  

  

  