﻿4 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  G7 
  

  

  from 
  Arroyo 
  Miguelete, 
  Montevideo, 
  Uruguay 
  (Cat. 
  No. 
  270908, 
  

   U.S.N.M.), 
  the 
  fugacious 
  periostracum 
  persists 
  over 
  a 
  large 
  portion 
  

   of 
  the 
  shell, 
  making 
  this 
  portion 
  appear 
  as 
  if 
  covered 
  with 
  very 
  

   thin 
  dead 
  skin. 
  On 
  the 
  portion 
  where 
  the 
  fugacious 
  periostracum 
  

   has 
  disappeared 
  the 
  surface 
  has 
  the 
  appearance 
  most 
  usually 
  seen 
  

   in 
  this 
  species. 
  In 
  Spatha 
  toahlbergi 
  of 
  South 
  Africa 
  the 
  bloom 
  

   persists 
  in 
  only 
  a 
  few 
  spots 
  and 
  is 
  so 
  thin 
  that 
  it 
  forms 
  but 
  the 
  thin- 
  

   nest 
  of 
  films. 
  In 
  Anodontites 
  tenebricosm 
  Lea 
  it 
  is 
  much 
  thicker, 
  

   more 
  easily 
  visible, 
  and 
  sometimes 
  remains 
  over 
  a 
  large 
  area. 
  In 
  

   M 
  onocondylaea 
  it 
  becomes 
  somewhat 
  like 
  pale 
  yellowish 
  or 
  whitish 
  

   paper, 
  and 
  in 
  many 
  places 
  where 
  it 
  has 
  partly 
  torn 
  away 
  from 
  the 
  

   shell 
  it 
  stands 
  up 
  on 
  the 
  surface 
  in 
  little 
  concentric 
  plates. 
  This 
  

   is 
  what 
  gives 
  M 
  onocondylaea 
  the 
  generally 
  roughened 
  appearance 
  

   so 
  often 
  noted 
  and 
  sometimes 
  mentioned 
  in 
  descriptions 
  as 
  being 
  

   lamellate. 
  In 
  this 
  genus 
  there 
  are 
  numerous 
  cracks 
  arranged 
  con- 
  

   centrically, 
  with 
  many 
  cross 
  cracks 
  uniting 
  them. 
  In 
  many 
  of 
  these 
  

   cracks 
  the 
  fugacious 
  periostracum 
  persists 
  throughout 
  the 
  life 
  of 
  

   the 
  shell. 
  This 
  makes 
  M 
  onocondylaea 
  one 
  of 
  the 
  best 
  genera 
  for 
  

   examination 
  in 
  a 
  study 
  of 
  radiating 
  threads, 
  as 
  they 
  are 
  almost 
  

   always 
  to 
  be 
  found 
  on 
  the 
  paper-like 
  fugacious 
  periostracum 
  re- 
  

   maining 
  in 
  the 
  cracks.- 
  In 
  some 
  spots 
  on 
  the 
  type 
  of 
  M 
  onocondylaea 
  

   felipponei 
  Marshall 
  little 
  sheets 
  of 
  this 
  kind 
  of 
  periostracum 
  still 
  

   lie 
  flat 
  and 
  apparently 
  loose 
  except 
  along 
  one 
  edge. 
  In 
  the 
  genus 
  

   Diplodontites 
  there 
  are 
  but 
  the 
  faintest 
  traces 
  of 
  a 
  fugacious 
  peri- 
  

   ostracum 
  of 
  any 
  kind. 
  Each 
  genus 
  seems 
  to 
  have 
  its 
  own 
  peculi- 
  

   arities 
  in 
  this 
  kind 
  of 
  periostracum. 
  

  

  In 
  giving 
  details 
  of 
  the 
  radiating 
  striae 
  of 
  each 
  genus 
  frequent 
  

   mention 
  is 
  made 
  of 
  the 
  sinulus 
  of 
  the 
  various 
  species, 
  and 
  a 
  few 
  

   general 
  remarks 
  concerning 
  this 
  feature 
  of 
  the 
  shell 
  may 
  well 
  be 
  

   made 
  here. 
  In 
  most 
  of 
  the 
  shells 
  of 
  the 
  family 
  Mutelidae 
  the 
  sinulus 
  

   is 
  distinctly 
  triangular, 
  but 
  in 
  a 
  few 
  cases 
  where 
  the 
  shells 
  have 
  a 
  

   very 
  elongated 
  form 
  the 
  sinulus, 
  too, 
  is 
  elongated, 
  and 
  its 
  triangular 
  

   shape 
  is 
  not 
  so 
  apparent. 
  In 
  Mycetopoda, 
  although 
  the 
  shell 
  is 
  

   elongated, 
  the 
  sinulus 
  is 
  distinctly 
  triangular. 
  The 
  Mutelidae 
  and 
  

   the 
  Aetheriidae 
  are 
  the 
  only 
  families 
  in 
  which 
  the 
  sinulus 
  is 
  typically 
  

   triangular. 
  The 
  latter 
  family 
  contains 
  the 
  three 
  genera, 
  Mulleria, 
  

   Aetheria, 
  and 
  Bartlettia. 
  None 
  of 
  these 
  has 
  radiating 
  striae 
  so 
  far 
  

   as 
  can 
  be 
  determined 
  at 
  this 
  time. 
  There 
  seems 
  to 
  be 
  no 
  doubt 
  as 
  

   to 
  some 
  relationship 
  between 
  Bartlettia 
  and 
  Mulleria 
  and 
  the 
  family 
  

   Mutelidae. 
  The 
  form 
  of 
  the 
  young 
  of 
  Bartlettia 
  and 
  Mulleria, 
  the 
  

   locality 
  (South 
  America) 
  in 
  which 
  the 
  two 
  genera 
  are 
  found, 
  the 
  

   type 
  of 
  sinulus, 
  and 
  the 
  texture 
  of 
  the 
  shell 
  seem 
  to 
  indicate 
  a 
  nearer 
  

   relationship 
  between 
  these 
  two 
  genera 
  and 
  the 
  Mutelidae 
  than 
  be- 
  

   tween 
  Aetheria 
  and 
  the 
  Mutelidae. 
  The 
  genus 
  Pseudodon 
  of 
  eastern 
  

   Asia 
  sometimes 
  has 
  a 
  triangular 
  sinulus. 
  It 
  will 
  be 
  discussed 
  in 
  

   connection 
  with 
  the 
  genus 
  M 
  onocondylaea. 
  

  

  