﻿6 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  Genera 
  BRAZZEA 
  and 
  ARTHROPTERON 
  

   No 
  material 
  available 
  for 
  examination 
  and 
  data 
  in 
  general 
  lacking. 
  

  

  Genus 
  PLEIODON 
  

  

  Plate 
  4, 
  fig. 
  2 
  

  

  In 
  this 
  genus 
  but 
  two 
  species 
  were 
  available 
  for 
  study, 
  Pleiodon 
  

   ovatus 
  Swainson 
  from 
  Senegal, 
  and 
  P. 
  speckii 
  from 
  Lake 
  Tanganyi- 
  

   ka. 
  The 
  latter 
  is 
  a 
  very 
  large 
  old 
  specimen 
  and 
  periostracal 
  

   characters 
  have 
  disappeared. 
  Of 
  P. 
  ovatus 
  there 
  are 
  two 
  rather 
  

   young 
  having 
  a 
  length 
  of 
  50 
  and 
  60 
  millimeters, 
  respectively. 
  These 
  

   two 
  specimens 
  have 
  the 
  radiating 
  threads 
  present 
  over 
  a 
  large 
  part 
  

   of 
  the 
  surface 
  and 
  they 
  are 
  perfectly 
  preserved 
  and 
  number 
  about. 
  

   105 
  to 
  the 
  millimeter. 
  There 
  are 
  also 
  seven 
  adult 
  specimens 
  of 
  this 
  

   species 
  in 
  the 
  collection, 
  the 
  largest 
  having 
  a 
  length 
  of 
  110 
  milli- 
  

   meters. 
  In 
  these 
  specimens 
  it 
  is 
  difficult 
  to 
  find 
  a 
  spot 
  in 
  which 
  

   threads 
  can 
  be 
  seen. 
  The 
  sinulus 
  in 
  this 
  genus 
  is 
  aberrent 
  from 
  the 
  

   Mutelid 
  type, 
  though 
  in 
  P. 
  speckii 
  it 
  approaches 
  it. 
  The 
  figure 
  (pi. 
  

   4, 
  fig. 
  2), 
  is 
  from 
  a 
  young 
  P. 
  ovatus 
  from 
  Senegal, 
  Africa 
  (Cat. 
  No. 
  

   86774, 
  U.S.N.M.). 
  The 
  broad 
  light-colored 
  band 
  near 
  the 
  bottom 
  

   of 
  the 
  figure 
  represents 
  the 
  remains 
  of 
  fugacious 
  periostracum 
  at- 
  

   tached 
  along 
  a 
  growth 
  line. 
  Other 
  remains 
  are 
  seen 
  at 
  the 
  left 
  of 
  

   the 
  figure. 
  

  

  Genus 
  MONOCONDYLAEA 
  

  

  Plate 
  1, 
  fig. 
  2; 
  plate 
  2, 
  fig. 
  1 
  

  

  In 
  nearly 
  every 
  specimen 
  of 
  this 
  genus 
  the 
  radiating 
  threads 
  

   persist 
  on 
  some 
  part 
  of 
  the 
  surface, 
  regardless 
  of 
  the 
  age 
  of 
  the 
  speci- 
  

   men. 
  Often 
  they 
  are 
  to 
  be 
  found 
  only 
  on 
  the 
  paper-like 
  remains 
  of 
  

   the 
  fugacious 
  periostracum 
  which 
  has 
  been 
  sheltered 
  in 
  the 
  peculiar 
  

   concentric 
  and 
  cross 
  cracks 
  nearly 
  always 
  found 
  in 
  these 
  shells. 
  In 
  

   M 
  onocondylaea 
  franciscana 
  Moricand 
  the 
  threads 
  occur 
  in 
  fine 
  condi- 
  

   tion, 
  number 
  about 
  85 
  to 
  the 
  millimeter, 
  and 
  are 
  found 
  over 
  nearly 
  the 
  

   whole 
  extent 
  of 
  the 
  shell. 
  On 
  the 
  single 
  specimen 
  of 
  M. 
  felipponei 
  

   Marshall 
  available 
  for 
  examination 
  the 
  threads 
  number 
  about 
  110 
  to 
  

   the 
  millimeter, 
  cover 
  nearly 
  the 
  whole 
  shell, 
  and 
  are 
  clearly 
  defined 
  

   and 
  easy 
  to 
  find. 
  M. 
  franciscana 
  is 
  figured 
  on 
  plate 
  1. 
  figure 
  2. 
  

   The 
  specimen 
  came 
  from 
  Rio 
  Francisco, 
  Brazil 
  (Cat. 
  No. 
  86334, 
  

   U.S.N.M.). 
  M. 
  felipponei 
  is 
  represented 
  on 
  plate 
  2, 
  figure 
  1. 
  It 
  

   came 
  from 
  Barra 
  del 
  Arroyo 
  Sacra, 
  Paysandu. 
  Uruguay 
  (Cat. 
  No. 
  

   340663, 
  U.S.N.M.). 
  

  

  Some 
  comparisons 
  between 
  the 
  Unionid 
  genus 
  Pseudodon 
  and 
  the 
  

   Mutelid 
  genus 
  M 
  onocondylaea 
  may 
  not 
  be 
  amiss. 
  The 
  shells 
  of 
  

   Pseudodon 
  present 
  some 
  peculiar 
  and 
  interesting 
  features. 
  The 
  

   genus 
  is 
  restricted 
  to 
  Eastern 
  Asia 
  and 
  some 
  of 
  the 
  near-by 
  islands. 
  

  

  