﻿Verrill 
  and 
  Bush 
  — 
  Genera 
  of 
  Zed 
  idee 
  and 
  Nuculidm. 
  51 
  

  

  Art. 
  Y. 
  — 
  Revision 
  of 
  the 
  Genera 
  of 
  Ledidce 
  and 
  Niicu- 
  

   Udo3 
  of 
  the 
  Atlantic 
  Coast 
  of 
  the 
  United 
  States 
  / 
  by 
  A. 
  E. 
  

   Yereill 
  and 
  Katharine 
  J. 
  Bush. 
  

  

  (Brief 
  Contributions 
  to 
  Zoology 
  from 
  the 
  Museum 
  of 
  Yale 
  University, 
  No. 
  L.) 
  

  

  A 
  somewhat 
  extended 
  study 
  of 
  the 
  series 
  of 
  deep-sea 
  bivalves 
  

   belonging 
  to 
  these 
  families, 
  dredged 
  off 
  our 
  coast 
  by 
  the 
  U. 
  

   S. 
  Fish 
  Commission, 
  from 
  1872 
  to 
  1887, 
  has 
  compelled 
  us 
  to 
  

   revise 
  the 
  known 
  genera 
  and 
  subgenera 
  and 
  to 
  propose 
  several 
  

   new 
  groups. 
  In 
  view 
  of 
  an 
  unexpected 
  delay 
  in 
  the 
  publica- 
  

   tion 
  of 
  the 
  report 
  upon 
  these 
  families, 
  which 
  had 
  been 
  com- 
  

   pleted 
  and 
  fully 
  illustrated, 
  it 
  seemed 
  desirable 
  to 
  publish 
  a 
  

   brief 
  preliminary 
  account 
  of 
  the 
  classification 
  adopted. 
  

  

  These 
  families 
  are 
  often 
  united 
  by 
  modern 
  malacologists 
  

   under 
  a 
  single 
  family 
  (Nuculidae), 
  while 
  others 
  regard 
  them 
  as 
  

   distinct. 
  They 
  are 
  certainly 
  closely 
  related 
  anatomically, 
  as 
  

   well 
  as 
  by 
  the 
  structure 
  of 
  the 
  shell. 
  Thus 
  all 
  the 
  members 
  

   of 
  both 
  families 
  have 
  a 
  single 
  pair 
  of 
  simple 
  " 
  foliobranchiate 
  " 
  

   (or 
  protobranchiate) 
  gills; 
  two 
  pairs 
  of 
  large 
  labial 
  palpi, 
  the 
  

   outer 
  ones 
  furnished 
  with 
  long 
  extensile 
  labial 
  tentacles 
  ; 
  a 
  

   large 
  muscular 
  foot 
  with 
  an 
  expanded, 
  concave, 
  terminal 
  disk, 
  

   adapted 
  for 
  rapid 
  motions 
  in 
  jumping 
  and 
  swimming, 
  as 
  well 
  

   as 
  for 
  creeping 
  ; 
  and 
  all 
  have 
  two 
  series 
  of 
  transverse 
  teeth 
  on 
  

   the 
  hinge-margin. 
  The 
  peculiar 
  structures 
  of 
  foot 
  and 
  gills 
  

   appear 
  together 
  elsewhere 
  only 
  in 
  the 
  family 
  Solemyidae, 
  

   which 
  is 
  evidently 
  a 
  related 
  group, 
  though 
  it 
  lacks 
  hinge 
  teeth. 
  

   As 
  these 
  three 
  families 
  have 
  gills 
  of 
  a 
  peculiar 
  and 
  simple 
  

   structure, 
  each 
  one 
  consisting 
  of 
  two 
  rows 
  of 
  flat 
  lamellae, 
  

   attached 
  to 
  a 
  single 
  stem, 
  they 
  have 
  recently 
  been 
  regarded 
  as 
  

   forming 
  a 
  special 
  order 
  (Protobranchiata). 
  

  

  This 
  group 
  is 
  of 
  special 
  interest 
  because 
  of 
  its 
  great 
  antiq- 
  

   uity. 
  Large 
  numbers 
  of 
  fossil 
  forms, 
  very 
  closely 
  allied 
  to 
  

   existing 
  genera 
  and 
  species, 
  occur 
  even 
  in 
  Silurian 
  and 
  Devon- 
  

   ian 
  formations. 
  

  

  Thus 
  the 
  common 
  living 
  genera 
  Nuoula 
  and 
  Leda 
  are 
  repre- 
  

   sented 
  by 
  numerous 
  Devonian 
  species, 
  many 
  of 
  which 
  cannot 
  

   be 
  separated 
  from 
  the 
  recent 
  forms, 
  even 
  as 
  subgenera, 
  by 
  any 
  

   tangible 
  characters. 
  Other 
  species 
  of 
  the 
  same 
  age, 
  referred 
  

   to 
  Palceoneilo, 
  agree 
  in 
  nearly 
  all 
  essential 
  characters 
  with 
  the 
  

   living 
  genus 
  Tindaria. 
  These 
  fossil 
  shells 
  are 
  generally 
  

   larger 
  and 
  stronger 
  than 
  the 
  corresponding 
  living 
  species. 
  

   Many 
  palaeozoic 
  genera 
  which 
  are 
  now 
  extinct 
  were 
  as 
  highly 
  

   organized 
  and 
  as 
  much 
  specialized 
  as 
  their 
  living 
  allies. 
  

  

  The 
  thin 
  shelled, 
  strongly 
  siphonate 
  genera, 
  such 
  as 
  Ybldia, 
  

   Yoldiella, 
  etc., 
  do 
  not 
  appear 
  so 
  early 
  in 
  geological 
  time 
  and 
  

  

  