﻿art. 
  4 
  SCULPTURE 
  OF 
  MUSSEL 
  SHELLS 
  MARSHALL 
  3 
  

  

  being: 
  found 
  as 
  far 
  north 
  as 
  Mexico 
  and 
  others 
  in 
  Central 
  America, 
  

   while 
  yet 
  others 
  are 
  found 
  in 
  South 
  America 
  only, 
  many 
  of 
  them 
  

   as 
  far 
  south 
  as 
  Rio 
  de 
  la 
  Plata, 
  and 
  one 
  species 
  as 
  far 
  as 
  Patagonia. 
  

  

  Of 
  the 
  two 
  genera 
  Brazzea 
  and 
  Arthropteron 
  no 
  material 
  was 
  at 
  

   hand 
  for 
  examination. 
  Of 
  the 
  other 
  12 
  genera 
  many 
  species 
  were 
  

   given 
  careful 
  scrutiny. 
  In 
  the 
  genus 
  Chelidonopsis, 
  of 
  which 
  only 
  

   three 
  specimens 
  were 
  available, 
  there 
  seems 
  to 
  be 
  no 
  sign 
  of 
  ra- 
  

   diating 
  threads. 
  In 
  the 
  genus 
  Mycetopoda 
  threads 
  were 
  found 
  

   on 
  only 
  a 
  couple 
  of 
  specimens 
  and 
  then 
  they 
  were 
  not 
  of 
  the 
  usual 
  

   type. 
  The 
  other 
  10 
  genera 
  all 
  showed 
  the 
  threads 
  clearly 
  in 
  most 
  

   of 
  the 
  species. 
  The 
  threads 
  in 
  the 
  African 
  genera 
  Spatha 
  and 
  

   Mutela 
  are 
  much 
  finer 
  and 
  more 
  numerous 
  than 
  in 
  the 
  American 
  

   genera, 
  but 
  the 
  African 
  genus 
  Pleiodon 
  has 
  threads 
  which 
  are 
  al- 
  

   most 
  exactly 
  like 
  those 
  of 
  the 
  South 
  American 
  genus 
  M 
  onocondylaea. 
  

  

  In 
  general 
  it 
  may 
  be 
  said 
  that 
  the 
  radiating 
  striae 
  resemble 
  the 
  

   threads 
  in 
  finely 
  woven 
  serge 
  cloth 
  when 
  it 
  is 
  viewed 
  with 
  the 
  naked 
  

   eye, 
  or 
  perhaps 
  it 
  would 
  be 
  better 
  to 
  say 
  that 
  they 
  resemble 
  the 
  

   fine 
  ridges 
  which 
  occur 
  on 
  our 
  finger 
  tips. 
  When 
  viewed 
  under 
  the 
  

   microscope 
  the 
  shells 
  whose 
  periostracum 
  retains 
  the 
  threads 
  look 
  

   as 
  if 
  they 
  had 
  been 
  marked 
  with 
  fingerprints. 
  

  

  In 
  some 
  cases 
  the 
  radiating 
  threads 
  are 
  very 
  clear 
  and 
  can 
  be 
  

   found 
  on 
  all 
  parts 
  of 
  the 
  shell. 
  Diplodontites 
  cookei 
  and 
  Mono- 
  

   condylaea 
  franclscana 
  are 
  notable 
  in 
  this 
  respect. 
  In 
  other 
  cases 
  the 
  

   threads 
  have 
  disappeared 
  from 
  most 
  of 
  the 
  shell, 
  and 
  sometimes 
  

   there 
  are 
  but 
  very 
  small 
  patches 
  of 
  threads 
  left 
  here 
  and 
  there. 
  Fre- 
  

   quently 
  it 
  is 
  necessary 
  to 
  make 
  a 
  very 
  careful 
  search 
  over 
  the 
  whole 
  

   surface 
  of 
  a 
  number 
  of 
  specimens 
  in 
  order 
  to 
  find 
  a 
  spot 
  in 
  which 
  

   the 
  striae 
  have 
  been 
  preserved. 
  In 
  some 
  of 
  the 
  groups 
  of 
  large 
  Ano- 
  

   dontites 
  typified 
  b}^ 
  trapesialis 
  (containing 
  jeivettianus, 
  forbesianus, 
  

   gJaucus, 
  and 
  others) 
  no 
  striae 
  have 
  thus 
  far 
  been 
  observed. 
  It 
  is 
  not 
  

   possible 
  to 
  say 
  at 
  this 
  time 
  whether 
  threads 
  are 
  lacking 
  in 
  these 
  shells 
  

   or 
  have 
  been 
  worn 
  away 
  or 
  lost 
  in 
  the 
  shedding 
  of 
  a 
  fugacious 
  

   periostracum. 
  In 
  Mycetopoda 
  the 
  threads 
  are 
  not 
  of 
  the 
  usual 
  type. 
  

   This 
  genus 
  will 
  be 
  discussed 
  later 
  in 
  this 
  paper 
  in 
  dealing 
  with 
  the 
  

   species 
  of 
  shell 
  which 
  is 
  called 
  Solenaia 
  falcata 
  Higgins. 
  

  

  In 
  the 
  preceding 
  paragraph 
  reference 
  has 
  been 
  made 
  to 
  a 
  fuga- 
  

   cious 
  periostracum. 
  Some 
  explanation 
  of 
  this 
  kind 
  of 
  periostracum 
  

   is 
  advisable, 
  as 
  its 
  presence 
  is 
  not 
  generally 
  known. 
  Very 
  often 
  

   there 
  is 
  a 
  sort 
  of 
  bloom 
  found 
  in 
  spots, 
  or 
  sometimes 
  covering 
  a 
  

   large 
  portion 
  of 
  the 
  shell. 
  Perhaps 
  it 
  has 
  generally 
  been 
  mistaken 
  

   tor 
  a 
  deposit 
  of 
  some 
  extraneous 
  material. 
  It 
  seems 
  to 
  be 
  a 
  part 
  

   of 
  the 
  periostracum. 
  When 
  present 
  the 
  bloom 
  is 
  likely 
  to 
  show 
  the 
  

   microscopic 
  threads 
  more 
  clearly 
  than 
  do 
  the 
  parts 
  of 
  the 
  shell 
  

   from 
  which 
  the 
  bloom 
  has 
  disappeared. 
  It 
  appears 
  to 
  be 
  usually 
  

   only 
  temporary. 
  On 
  a 
  specimen 
  of 
  Anodontites 
  tcnebricosus 
  Lea 
  

  

  