﻿NO. 
  1139. 
  BEEF-WATER 
  MOLLUSCA—VERBILL 
  AND 
  BUSH. 
  849 
  

  

  simple 
  "foliobranchiate'' 
  (or 
  protobraucliiate) 
  gills; 
  two 
  pairs 
  of 
  large 
  

   labial 
  palpi, 
  the 
  outer 
  ones 
  furnished 
  with 
  long 
  extensile 
  labial 
  tenta- 
  

   cles; 
  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 
  or 
  obli(|ue 
  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 
  and 
  has 
  a 
  very 
  diiierent 
  shell. 
  

   As 
  these 
  three 
  families 
  have 
  gills 
  of 
  a 
  peculiar 
  and 
  simple 
  structure, 
  

   each 
  one 
  consisting 
  of 
  two 
  rows 
  of 
  flat 
  lamelliTe, 
  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 
  antiquity. 
  

   Large 
  numbers 
  of 
  fossil 
  forms 
  very 
  closely 
  allied 
  to 
  existing 
  genera 
  

   and 
  species 
  occur 
  even 
  in 
  Silurian 
  and 
  Devonian 
  formations. 
  

  

  Thus 
  the 
  common 
  living 
  genera 
  Xucida 
  and 
  Leda 
  are 
  represented 
  by 
  

   numerous 
  Devonian 
  species, 
  many 
  of 
  which 
  can 
  not 
  be 
  separated 
  from 
  

   the 
  recent 
  forms, 
  even 
  as 
  subgenera, 
  by 
  any 
  tangible 
  characters. 
  Other 
  

   species 
  of 
  the 
  same 
  age, 
  referred 
  to 
  Palwoneilo, 
  agree 
  in 
  nearly 
  all 
  essen- 
  

   tial 
  characters 
  with 
  the 
  living 
  genus 
  Tindaria. 
  These 
  fossil 
  shells 
  are 
  

   generally 
  larger 
  and 
  stronger 
  than 
  the 
  corresponding 
  living 
  species. 
  

   Many 
  Paheozoic 
  genera 
  which 
  are 
  now 
  extinct 
  were 
  as 
  highly 
  organized 
  

   and 
  as 
  innch 
  specialized 
  as 
  their 
  living 
  allies. 
  

  

  The 
  thin-shelled, 
  strongly 
  siphonate 
  genera, 
  such 
  as 
  Toldia, 
  Toldi- 
  

   ella, 
  etc., 
  do 
  not 
  ai)pear 
  so 
  early 
  in 
  geological 
  time 
  and 
  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 
  move- 
  

   ments, 
  in 
  which 
  a 
  heav^y 
  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 
  SoJemya 
  the 
  shell 
  is 
  still 
  lighter 
  and 
  thinner, 
  in 
  

   accordance 
  with 
  more 
  developed 
  swimming 
  habits, 
  combined 
  with 
  bur- 
  

   rowing 
  when 
  at 
  rest. 
  Such 
  forms 
  as 
  Xucula 
  and 
  Tindaria, 
  which 
  have 
  

   no 
  siphon 
  tubes, 
  must 
  live 
  at 
  or 
  near 
  the 
  surface 
  of 
  the 
  mud, 
  ov^er 
  which 
  

   they 
  creep 
  with 
  their 
  large 
  expanded 
  pedal 
  disk. 
  These 
  have, 
  for 
  their 
  

   j)rotection, 
  comparatively 
  solid 
  shells 
  similar 
  to 
  those 
  of 
  Palaeozoic 
  spe- 
  

   cies, 
  in 
  form, 
  texture, 
  and 
  sculpture. 
  

  

  The 
  family 
  Nuculidte 
  differs 
  from 
  Ledid* 
  mainly 
  in 
  having 
  no 
  siphon 
  

   tubes, 
  the 
  mantle 
  edges 
  being 
  completely 
  disunited. 
  The 
  Ledidte 
  are 
  

   remarkable 
  for 
  the 
  great 
  variations 
  in 
  the 
  structure 
  of 
  the 
  hinge-teeth, 
  

   ligament, 
  cartilage, 
  and 
  mantle, 
  as 
  well 
  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 
  

   separated 
  [Leda) 
  ; 
  while 
  in 
  Tindaria. 
  there 
  is 
  no 
  true 
  siphon, 
  but 
  only 
  an 
  

   Proc. 
  ]S^. 
  M. 
  vol. 
  XX 
  54 
  

  

  