﻿2 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  no 
  mention 
  has 
  ever 
  been 
  made 
  of 
  the 
  microscopic 
  features 
  of 
  the 
  

   periostracum 
  which 
  are 
  shown 
  so 
  clearly 
  in 
  many 
  of 
  the 
  species. 
  

   Anodontites 
  crispatus 
  Bruguiere, 
  described 
  in 
  1792, 
  the 
  type 
  of 
  the 
  

   genus, 
  has 
  sculpture 
  nearly 
  as 
  fine 
  and 
  beautiful 
  as 
  that 
  of 
  Diplo- 
  

   dontites 
  cookei 
  and 
  yet 
  that 
  fact 
  has 
  remained 
  unrevealed 
  for 
  a 
  

   period 
  of 
  132 
  years. 
  In 
  the 
  genus 
  Monocondylaca 
  the 
  two 
  species 
  M. 
  

   paraguayana 
  Orbigny 
  and 
  M 
  . 
  franciscana 
  Moricand 
  were 
  described 
  

   in 
  1835 
  and 
  1837, 
  respectively. 
  Both 
  species 
  show 
  the 
  microscopic, 
  

   radiating 
  striae, 
  the 
  latter 
  especially 
  having 
  them 
  in 
  unusual 
  per- 
  

   fection. 
  Apparently 
  no 
  mention 
  of 
  them 
  has 
  ever 
  been 
  made 
  until 
  

   the 
  present 
  time. 
  

  

  A 
  somewhat 
  parallel 
  case, 
  though 
  relating 
  to 
  a 
  different 
  style 
  of 
  

   sculpture, 
  is 
  presented 
  by 
  the 
  Chinese 
  genus 
  Schistodesmus, 
  which 
  

   possesses 
  a 
  concentric 
  sculpture 
  of 
  microscopic 
  striae 
  wonderfully 
  

   fine 
  and 
  beautiful. 
  Baird 
  and 
  Adams 
  (1867) 
  in 
  their 
  description 
  

   of 
  the 
  species 
  lampreyanus 
  failed 
  to 
  mention 
  them, 
  and 
  they 
  seem 
  

   to 
  have 
  escaped 
  any 
  notice 
  until 
  1900, 
  when 
  Simpson, 
  in 
  his 
  descrip- 
  

   tion 
  of 
  the 
  genus 
  Schistodesmus, 
  called 
  attention 
  to 
  them 
  thus: 
  

   " 
  Marvelously 
  delicate, 
  concentric, 
  microscopic 
  lirae." 
  The 
  genus 
  

   ^uneopsis, 
  also 
  of 
  China, 
  has 
  a 
  similar 
  sculpture, 
  though 
  on 
  a 
  cloth- 
  

   like 
  periostracum, 
  and 
  it 
  seems 
  that 
  these 
  two 
  genera 
  should 
  stand 
  

   next 
  to 
  each 
  other 
  instead 
  of 
  being 
  separated 
  by 
  the 
  genus 
  Gibbosula, 
  

   which 
  Simpson 
  has 
  placed 
  between 
  them. 
  

  

  As 
  has 
  already 
  been 
  said, 
  an 
  examination 
  has 
  been 
  made 
  of 
  the 
  

   microscopic 
  sculpture 
  of 
  shells 
  of 
  practically 
  all 
  the 
  genera 
  of 
  naiads 
  

   from 
  all 
  parts 
  of 
  the 
  world. 
  So 
  far 
  as 
  those 
  of 
  the 
  Unionidae 
  are 
  

   concerned, 
  not 
  much 
  may 
  be 
  said 
  at 
  present. 
  For 
  our 
  immediate 
  

   purpose 
  it 
  is 
  sufficient 
  to 
  say 
  that 
  in 
  this 
  family 
  regularly 
  arranged 
  

   microscopic 
  details 
  are 
  usually 
  lacking, 
  and 
  none 
  of 
  them 
  has 
  a 
  

   periostracum 
  made 
  up 
  of 
  fine 
  radiating 
  threads. 
  With 
  the 
  naiads 
  

   of 
  the 
  family 
  Mutelidae 
  the 
  case 
  is 
  different. 
  Here 
  many 
  species 
  

   have 
  an 
  almost 
  infinite 
  number 
  of 
  radiating 
  threads, 
  and 
  while 
  the 
  

   threads 
  seem 
  to 
  be 
  absent 
  in 
  a 
  few 
  species 
  it 
  is 
  believed 
  that 
  with 
  

   good 
  material 
  every 
  species 
  belonging 
  to 
  this 
  family 
  would 
  reveal 
  

   this 
  type 
  of 
  periostracum 
  and 
  that 
  it 
  is 
  a 
  family 
  characteristic. 
  

  

  This 
  peculiar 
  periostracum 
  is 
  so 
  striking 
  in 
  many 
  of 
  the 
  genera 
  

   and 
  species 
  of 
  the 
  family 
  that 
  if 
  it 
  be 
  shown 
  that 
  any 
  species 
  ab- 
  

   solutely 
  lacks 
  it 
  then 
  the 
  right 
  of 
  that 
  species 
  to 
  a 
  place 
  in 
  the 
  

   Mutelidae 
  becomes 
  subject 
  to 
  some 
  doubt. 
  

  

  The 
  family 
  Mutelidae 
  as 
  at 
  present 
  understood 
  contains 
  14 
  genera, 
  

   of 
  which 
  6 
  are 
  restricted 
  to 
  Africa, 
  namely, 
  Spatha, 
  Mutela, 
  Cheli- 
  

   donopsis, 
  Brazzea, 
  Arthropteron, 
  and 
  Pleiodon, 
  while 
  6 
  are 
  re- 
  

   stricted 
  to 
  South 
  America, 
  namely, 
  Monocondylaea, 
  Iheringella, 
  

   Fos&ida, 
  Leila, 
  Mycetopoda, 
  and 
  Diplodontites. 
  Anodontites, 
  the 
  

   largest 
  genus 
  of 
  Mutelidae, 
  is 
  restricted 
  to 
  America, 
  some 
  species 
  

  

  