﻿art. 
  4 
  SCULPTURE 
  OF 
  MUSSEL 
  SHELLS 
  MARSHALL 
  7 
  

  

  The 
  shells 
  have 
  an 
  apparently 
  near 
  relationship 
  to 
  some 
  of 
  the 
  

   South 
  American 
  naiads, 
  especially 
  to 
  those 
  of 
  the 
  genus 
  Monocondy- 
  

   laea, 
  because 
  of 
  the 
  single 
  cardinal 
  tooth 
  and 
  the 
  general 
  character 
  

   of 
  the 
  sinulus 
  in 
  some 
  of 
  the 
  Ps< 
  udodon 
  species. 
  Because 
  of 
  these 
  

   features, 
  several 
  species 
  of 
  Pseudodon 
  were 
  described 
  as 
  Monocon- 
  

   dylaea, 
  and 
  many 
  years 
  passed 
  before 
  it 
  became 
  generally 
  recognized 
  

   that 
  the 
  apparent 
  close 
  relationship 
  of 
  the 
  two 
  genera 
  comes 
  from 
  

   a 
  superficial 
  resemblance 
  rather 
  than 
  from 
  structural 
  affinities. 
  As 
  

   time 
  passed 
  the 
  shells 
  were 
  not 
  only 
  placed 
  in 
  different 
  genera, 
  but 
  

   as 
  they 
  became 
  more 
  fully 
  understood 
  they 
  were 
  classified 
  into 
  

   different 
  families, 
  Pseudodon 
  in 
  the 
  Unionidae 
  and 
  Monocondylaea 
  

   in 
  the 
  Mutelidae. 
  The 
  collection 
  of 
  the 
  United 
  States 
  National 
  Mu- 
  

   seum 
  contains 
  many 
  specimens 
  representing 
  eleven 
  species 
  of 
  Pseudo- 
  

   don. 
  All 
  of 
  these 
  have 
  been 
  subjected 
  to 
  searching 
  microscopical 
  

   examination 
  to 
  determine 
  the 
  presence 
  or 
  absence 
  of 
  the 
  radiating 
  

   threads 
  characteristic 
  of 
  Monocondylaea 
  and 
  other 
  Mutelidae. 
  No 
  

   trace 
  of 
  such 
  threads 
  was 
  found 
  in 
  any 
  species. 
  Their 
  absence 
  

   affords 
  additional 
  evidence 
  of 
  the 
  lack 
  of 
  any 
  very 
  close 
  relationship 
  

   between 
  Pseudodon 
  and 
  Monocondylaea. 
  

  

  As 
  has 
  been 
  said 
  above, 
  the 
  sinulus 
  of 
  Pseudodon 
  often 
  resembles 
  

   that 
  of 
  the 
  Mutelidae. 
  In 
  some 
  species 
  of 
  Pseudodon 
  the 
  resem- 
  

   blance 
  is 
  quite 
  sharp, 
  but 
  in 
  others 
  it 
  is 
  not 
  clear 
  or 
  is 
  lacking. 
  

   Even 
  in 
  the 
  cases 
  in 
  which 
  it 
  is 
  most 
  striking 
  (as 
  in 
  Pseudodon 
  

   cambojensis 
  Petit, 
  P. 
  polita 
  Mousson, 
  and 
  P. 
  cumingii 
  Lea) 
  it 
  lacks 
  

   the 
  sharply 
  equilaterally 
  triangular 
  form 
  of 
  the 
  sinulus 
  of 
  the 
  Mute- 
  

   lidae, 
  being 
  more 
  or 
  less 
  rounded 
  at 
  the 
  lower 
  point. 
  Some 
  of 
  the 
  

   other 
  species 
  of 
  Pseudodon, 
  such 
  as 
  P. 
  loomisi 
  Simpson 
  and 
  P. 
  

   crebristriatus 
  Anthony, 
  have 
  the 
  sinulus 
  as 
  in 
  the 
  other 
  Unionidae. 
  

  

  Genus 
  IHERINGELLA 
  

  

  Of 
  this 
  genus, 
  which 
  shows 
  an 
  intimate 
  relationship 
  to 
  Mono- 
  

   condylaea, 
  but 
  one 
  specimen 
  was 
  available. 
  It 
  is 
  Iheringella 
  iso- 
  

   cardioides 
  Lea 
  (Cat. 
  No. 
  86326, 
  U.S.N.M.), 
  and 
  comes 
  from 
  the 
  

   Rio 
  de 
  la 
  Plata, 
  South 
  America. 
  While 
  it 
  is 
  in 
  rather 
  poor 
  con- 
  

   dition, 
  fortunately 
  a 
  few 
  small 
  spots 
  are 
  well 
  enough 
  preserved 
  to 
  

   show 
  that 
  the 
  radiating 
  threads 
  occur 
  in 
  this 
  genus. 
  The 
  striae 
  do 
  

   not 
  show 
  sufficiently 
  well 
  to 
  photograph. 
  

  

  Genus 
  FOSSICULA 
  

  

  Plate 
  2, 
  fig. 
  2 
  

  

  But 
  one 
  of 
  the 
  two 
  species 
  was 
  available; 
  namely, 
  Fossicida 
  

   fossiculifera 
  Lea, 
  represented 
  by 
  four 
  specimens, 
  three 
  of 
  which 
  

   came 
  from 
  the 
  Parana 
  River 
  and 
  one 
  from 
  Piricicaba, 
  Sao 
  Paulo, 
  

   Brazil. 
  This 
  is 
  a 
  peculiar 
  genus 
  whose 
  relationships 
  point 
  in 
  two 
  

   directions 
  — 
  to 
  Monocondylaea, 
  because 
  of 
  the 
  cardinal 
  tooth, 
  and 
  to 
  

  

  