﻿52 
  Verrill 
  and 
  Bush 
  — 
  Genera 
  of 
  Ledidm 
  and 
  Nuculidai. 
  

  

  may 
  be 
  regarded 
  as 
  more 
  modern 
  specializations 
  of 
  the 
  Leda- 
  

   like 
  forms. 
  They 
  are 
  also 
  the 
  forms 
  that 
  swim 
  and 
  jump 
  with 
  

   the 
  greatest 
  activity. 
  Therefore 
  the 
  thin 
  and 
  light 
  character 
  

   of 
  their 
  shells 
  may 
  be 
  regarded 
  as 
  having 
  been 
  secondarily 
  

   acquired, 
  partly 
  in 
  consequence 
  of 
  their 
  active 
  movements, 
  in 
  

   which 
  a 
  heavy 
  shell 
  would 
  be 
  disadvantageous, 
  and 
  partly 
  

   because 
  the 
  development 
  of 
  long 
  siphons 
  enables 
  them 
  to 
  live 
  

   concealed, 
  much 
  of 
  the 
  time, 
  beneath 
  the 
  surface 
  of 
  the 
  soft 
  

   mud 
  in 
  which 
  they 
  generally 
  live. 
  In 
  Solemya 
  the 
  shell 
  is 
  

   still 
  lighter 
  and 
  thinner, 
  in 
  accordance 
  with 
  more 
  developed 
  

   swimming 
  habits, 
  combined 
  with 
  burrowing 
  when 
  at 
  rest. 
  

   Such 
  forms 
  as 
  Nucula 
  and 
  Tindaria, 
  which 
  have 
  no 
  siphon 
  

   tubes, 
  must 
  live 
  at 
  or 
  near 
  the 
  surface 
  of 
  the 
  mud 
  over 
  which 
  

   they 
  creep 
  with 
  their 
  large 
  expanded 
  pedal 
  disk. 
  (Fig. 
  15.) 
  

   These 
  have 
  for 
  their 
  protection 
  comparatively 
  solid 
  shells, 
  

   similar 
  to 
  those 
  of 
  palaeozoic 
  species, 
  in 
  form, 
  texture, 
  and 
  

   sculpture. 
  

  

  The 
  family 
  Nuculidse 
  differs 
  from 
  Led 
  idee 
  mainly 
  in 
  having 
  

   no 
  siphon 
  tubes, 
  the 
  mantle 
  edges 
  being 
  completely 
  disunited. 
  

   The 
  Ledidse 
  are 
  remarkable 
  for 
  the 
  great 
  variations 
  in 
  the 
  

   structure 
  of 
  the 
  hinge-teeth, 
  ligament, 
  cartilage, 
  and 
  mantle, 
  

   as 
  w 
  T 
  ell 
  as 
  in 
  the 
  form 
  of 
  the 
  shell. 
  The 
  pallial 
  sinus 
  may 
  be 
  

   wanting 
  or 
  well-developed. 
  Some 
  genera 
  have 
  long 
  united 
  

   siphons 
  ( 
  Yoldia) 
  / 
  some 
  have 
  shorter 
  ones, 
  more 
  or 
  less 
  sep- 
  

   arated 
  (Leda) 
  ; 
  while 
  in 
  Tindaria 
  there 
  is 
  no 
  true 
  siphon, 
  

   but 
  only 
  an 
  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 
  " 
  resi- 
  

   lium 
  " 
  ; 
  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 
  

   lamelliform 
  (Silicula). 
  The 
  beaks 
  generally 
  turn 
  backward 
  

   {Yoldia, 
  Leda, 
  Nucula), 
  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 
  diffi- 
  

   cult, 
  if 
  not 
  impossible, 
  to 
  tell 
  which 
  is 
  the 
  anterior 
  end 
  of 
  the 
  

   shell, 
  without 
  the 
  soft 
  parts. 
  Hence 
  many 
  fossil 
  and 
  some 
  

   recent 
  species 
  have 
  possibly 
  been 
  reversed 
  in 
  the 
  descriptions. 
  

   Thus 
  many 
  of 
  the 
  palaeozoic 
  species 
  referred 
  to 
  Nucula 
  are 
  

   described 
  as 
  having 
  the 
  beaks 
  turned 
  forward, 
  the 
  larger 
  end 
  

   of 
  the 
  shell 
  being 
  considered 
  posterior 
  ; 
  but 
  in 
  modern 
  Nucidai, 
  

   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 
  diffi- 
  

   culty. 
  When 
  a 
  differentiated 
  external 
  ligament 
  is 
  present, 
  we 
  

   have 
  assumed 
  that 
  it 
  is 
  posterior 
  to 
  the 
  beaks 
  (opisthodetic), 
  

  

  