﻿850 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  vol.xx. 
  , 
  

  

  efferent 
  orifice 
  differentiated. 
  The 
  ligament 
  may 
  be 
  wholly 
  external, 
  

   as 
  in 
  Malletia, 
  Tindaria, 
  etc., 
  or 
  it 
  may 
  be 
  rudimentary 
  and 
  replaced 
  

   by 
  an 
  internal 
  cartilage 
  or 
  "resilium," 
  or 
  both 
  may 
  coexist 
  in 
  varying 
  

   degrees 
  of 
  development 
  and 
  degeneration. 
  The 
  hinge- 
  teeth 
  may 
  be 
  very 
  

   numerous 
  and 
  regularly 
  V-shaped 
  in 
  each 
  series, 
  or 
  they 
  may 
  be 
  com- 
  

   paratively 
  few 
  and 
  irregular, 
  sometimes 
  becoming 
  oblique 
  and 
  lamelli- 
  

   form 
  {Silicida). 
  The 
  beaks 
  generally 
  turn 
  backward 
  {Yoldia, 
  Leda, 
  

   ¥ucula), 
  but 
  in 
  Malletia, 
  Tindaria, 
  and 
  some 
  other 
  genera 
  they 
  turn 
  

   forward. 
  On 
  this 
  account, 
  when 
  there 
  is 
  neither 
  pallial 
  sinus 
  nor 
  

   external 
  ligament, 
  it 
  is 
  often 
  difhcult, 
  if 
  not 
  imi^ossible, 
  to 
  tell 
  which 
  is 
  

   the 
  anterior 
  end 
  of 
  the 
  shell 
  without 
  the 
  soft 
  jiarts. 
  Hence 
  many 
  fos- 
  

   sil 
  and 
  some 
  recent 
  species 
  have 
  probably 
  been 
  reversed 
  in 
  the 
  descrip- 
  

   tions. 
  Thus 
  many 
  of 
  the 
  Paliieozoic 
  species 
  referred 
  to 
  Nucula 
  are 
  

   described 
  as 
  having 
  the 
  beaks 
  turned 
  forward, 
  the 
  longer 
  end 
  of 
  the 
  

   shell 
  being 
  considered 
  posterior, 
  but 
  in 
  modern 
  asucuUl' 
  the 
  beaks 
  turn 
  

   backward 
  and 
  the 
  shorter 
  end 
  is 
  posterior. 
  Many 
  of 
  the 
  deep-sea 
  

   species 
  with 
  small, 
  thin 
  shells 
  show 
  no 
  distinct 
  muscular 
  nor 
  pallial 
  

   scars, 
  which 
  increases 
  this 
  difficulty. 
  When 
  a 
  differentiated 
  external 
  

   ligament 
  is 
  present, 
  we 
  have 
  assumed 
  that 
  it 
  is 
  posterior 
  to 
  the 
  beaks 
  

   (opisthodetic), 
  though 
  a 
  narrow 
  extension 
  usually 
  runs 
  under 
  and 
  for- 
  

   ward 
  of 
  the 
  beaks 
  in 
  a 
  groove. 
  When 
  the 
  shell 
  of 
  a 
  dimyarian 
  bivalve 
  

   gapes 
  posteriorly, 
  the 
  existence 
  of 
  a 
  siphon 
  may 
  generally 
  be 
  assumed; 
  

   for 
  otherwise 
  the 
  internal 
  soft 
  parts 
  would 
  be 
  exposed 
  to 
  enemies. 
  The 
  

   existence 
  of 
  a 
  posterior 
  rostrum 
  or 
  a 
  protrusion 
  of 
  the 
  posterior 
  margin 
  

   defined 
  by 
  an 
  inferior 
  emargination 
  indicates 
  the 
  existence 
  of 
  a 
  siphon, 
  

   or 
  at 
  least 
  an 
  anal 
  tube, 
  but 
  these 
  organs 
  may 
  exist 
  without 
  such 
  mod- 
  

   ifications 
  of 
  the 
  shell. 
  If 
  these 
  rules 
  be 
  applied 
  to 
  Paleozoic 
  forms 
  we 
  

   must 
  conclude 
  that 
  the 
  rostrate 
  and 
  subrostrate 
  forms 
  of 
  Palceoneilo, 
  

   etc., 
  had 
  some 
  sort 
  of 
  a 
  siphon, 
  and 
  therefore 
  were 
  not 
  true 
  Kuculida3. 
  

  

  Numerous 
  Palaeozoic 
  species 
  referred 
  to 
  the 
  genus 
  Pakeoneilo 
  proba- 
  

   bly 
  belong 
  to 
  or 
  near 
  the 
  Tindarinai. 
  Some 
  of 
  the 
  species 
  ^ 
  from 
  the 
  

   American 
  Devonian 
  rocks 
  can 
  hardly 
  be 
  distinguished 
  from 
  Tindaria 
  

   by 
  any 
  important 
  structural 
  characters, 
  unless 
  it 
  be 
  the 
  form 
  of 
  the 
  

   teeth. 
  It 
  is 
  probable 
  that 
  Wueulites 
  and 
  several 
  related 
  genera 
  belong 
  

   near 
  this 
  division, 
  for 
  they 
  have 
  an 
  external 
  ligament 
  and 
  no 
  resilium. 
  

   In 
  these 
  genera 
  the 
  plain, 
  transverse 
  teeth 
  are 
  very 
  numerous 
  and 
  

   more 
  simple 
  than 
  in 
  the 
  modern 
  genera, 
  seldom 
  showing 
  any 
  trace 
  of 
  

   the 
  acute, 
  V-shaped 
  form 
  characteristic 
  of 
  most 
  modern 
  genera, 
  though 
  

   in 
  some 
  species 
  the 
  teeth 
  are 
  slightly 
  angulated 
  in 
  the 
  middle. 
  

  

  Mr. 
  Dall 
  has 
  proposed 
  the 
  family 
  Ctenodontida^ 
  - 
  to 
  include 
  numerous 
  

   Palaeozoic 
  species 
  belonging 
  to 
  Gtenodonta, 
  and 
  allied 
  genera, 
  some 
  of 
  

   which 
  Zittel 
  and 
  others 
  refer 
  to 
  Arcidiie 
  on 
  account 
  of 
  their 
  thickened 
  

   pectunculoid 
  shells. 
  They 
  seem 
  to 
  be 
  allied 
  rather 
  to 
  Tindarina?. 
  

  

  1 
  For 
  example 
  see 
  P. 
  constricta 
  Hall, 
  P. 
  plana 
  Hall 
  in 
  Palaeontology 
  of 
  New 
  York, 
  

   V, 
  Pt. 
  I, 
  pp. 
  333, 
  334, 
  pi. 
  XLviii, 
  figs. 
  1-28, 
  1885. 
  

   2Tran8. 
  Wagner 
  Free 
  Inst., 
  Ill, 
  p. 
  515, 
  1895 
  

  

  