﻿Yerrill 
  and 
  Bush 
  — 
  Genera 
  of 
  Ledidce 
  and 
  N'uculidce. 
  53 
  

  

  though 
  a 
  narrow 
  extension 
  usually 
  runs 
  under 
  and 
  forward 
  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 
  modifica- 
  

   tions 
  of 
  the 
  shell. 
  If 
  these 
  rules 
  be 
  applied 
  to 
  palaeozoic 
  forms 
  

   we 
  must 
  conclude 
  that 
  the 
  rostrate 
  and 
  subrostrate 
  forms 
  of 
  

   JPalmoneilo, 
  etc., 
  had 
  some 
  sort 
  of 
  a 
  siphon, 
  and 
  therefore 
  were 
  

   not 
  Nuculidse, 
  as 
  now 
  restricted. 
  

  

  Family 
  Nuculid^:. 
  

  

  We 
  have 
  included 
  Nucidina 
  (D'Orb.) 
  in 
  the 
  Nuculidse 
  with 
  

   some 
  doubt, 
  because 
  authors 
  differ 
  as 
  to 
  its 
  structure. 
  It 
  is 
  said 
  

   that 
  its 
  ligament 
  is 
  wholly 
  external 
  ; 
  if 
  so, 
  it 
  should, 
  perhaps, 
  

   form 
  a 
  distinct 
  subfamily. 
  Its 
  anatomy 
  is 
  unknown. 
  Fischer 
  

   places 
  it 
  in 
  the 
  Arcidse, 
  near 
  Limopsis, 
  but 
  it 
  has 
  no 
  median 
  

   ligamental 
  area. 
  

  

  Subfamily 
  Glomin^e, 
  nov. 
  

  

  Ligament 
  thick, 
  elongated, 
  attached 
  for 
  most 
  of 
  its 
  length 
  to 
  

   the 
  inner 
  surface 
  of 
  the 
  posterior 
  hinge-plate 
  and 
  running 
  forward 
  

   in 
  a 
  narrow 
  groove 
  beneath 
  the 
  beaks, 
  so 
  that 
  its 
  anterior 
  portion 
  

   is 
  external 
  and 
  the 
  thickened 
  posterior 
  portion 
  is 
  partly 
  internal. 
  

   No 
  pallial 
  sinus. 
  Animal 
  not 
  known. 
  

  

  This 
  group 
  includes, 
  so 
  far 
  as 
  known, 
  only 
  the 
  genus 
  Glomus 
  

   Jeffreys, 
  which 
  has 
  been 
  referred 
  by 
  several 
  writers 
  to 
  the 
  Arcidse, 
  

   and 
  by 
  others 
  to 
  the 
  Ledidse, 
  from 
  both 
  of 
  which 
  it 
  differs 
  widely. 
  

   Its 
  relations 
  to 
  the 
  Nuculiclse 
  are 
  somewhat 
  uncertain, 
  owing 
  to 
  

   our 
  ignorance 
  of 
  the 
  soft 
  parts. 
  In 
  the 
  form 
  and 
  position 
  of 
  the 
  

   ligament 
  it 
  differs 
  entirely 
  from 
  all 
  other 
  genera 
  of 
  Nuculidse 
  and 
  

   Ledidse. 
  

  

  Glomus 
  Jeffreys. 
  Figures 
  1, 
  2. 
  

  

  Glomus 
  Jeffreys, 
  Annals, 
  Mag. 
  Nat. 
  Hist., 
  p. 
  433, 
  1876 
  ; 
  Proc. 
  

   Zool. 
  Soc. 
  London, 
  p. 
  573, 
  pi. 
  xlv, 
  figs. 
  5, 
  5a, 
  1879 
  ; 
  Smith, 
  Report 
  

   Voy. 
  Challenger, 
  Zool, 
  xiii, 
  pp. 
  248-249, 
  pi. 
  xxi, 
  figs. 
  1-3&, 
  1885. 
  

  

  Shell 
  thin, 
  smooth, 
  sub-equilateral, 
  rounded 
  at 
  both 
  ends, 
  with 
  

   the 
  beaks 
  turned 
  forward. 
  No 
  lunule 
  nor 
  escutcheon. 
  Hinge 
  

   with 
  two 
  series 
  of 
  obliquely 
  transverse 
  teeth; 
  a 
  small 
  lateral 
  

   tooth. 
  

  

  The 
  following 
  are 
  described 
  species 
  : 
  

  

  G. 
  nitens 
  Jeff, 
  G. 
  Jeffrey 
  si 
  Smith, 
  G. 
  simplex 
  Smith, 
  G. 
  ince- 
  

   quilateralis 
  Smith, 
  G. 
  Japonicus 
  Smith. 
  

  

  