﻿NO. 
  1139. 
  DEEP-WATEB 
  MOLLUSGA—VERBILL 
  AND 
  BUSH. 
  807 
  

  

  somewliat 
  upturned. 
  The 
  entire 
  body 
  of 
  the 
  shell 
  is 
  covered 
  with 
  

   numerous 
  narrow, 
  elevated, 
  radiating 
  ribs, 
  separated 
  by 
  much 
  wider 
  

   concave 
  interspaces, 
  some 
  of 
  the 
  widest 
  of 
  which 
  have 
  a 
  small 
  second- 
  

   ary 
  rib 
  in 
  the 
  center 
  toward 
  the 
  margin; 
  the 
  ribs 
  increase 
  in 
  elevation 
  

   and 
  strength 
  posteriorly, 
  toward 
  the 
  base 
  of 
  the 
  rostrum, 
  but 
  never 
  

   become 
  broad; 
  for 
  a 
  short 
  distance 
  on 
  the 
  base 
  of 
  the 
  rostrum 
  the 
  ribs 
  

   are 
  nearly 
  obsolete 
  but 
  become 
  prominent 
  again 
  on 
  its 
  dorsal 
  and 
  ter- 
  

   minal 
  portions; 
  this 
  part 
  is 
  also 
  crossed 
  by 
  irregular 
  raised 
  lines 
  of 
  

   growth 
  which 
  cross 
  the 
  ribs 
  obliquely 
  ; 
  the 
  inner 
  surface 
  is 
  covered 
  

   with 
  rounded 
  grooves 
  corresponding 
  to 
  the 
  external 
  ribs, 
  separated 
  

   by 
  convex 
  ribs 
  of 
  about 
  the 
  same 
  width; 
  these 
  become 
  obsolete 
  ante- 
  

   riorly 
  and 
  posteriorly. 
  The 
  hinge-margin 
  in 
  the 
  left 
  valve 
  is 
  only 
  a 
  

   little 
  thickened 
  and 
  slightly 
  excurved, 
  the 
  cartilage-plate 
  is 
  central, 
  

   stout, 
  regularly 
  ovate 
  in 
  form, 
  with 
  a 
  thickened 
  inner 
  margin 
  ; 
  in 
  the 
  

   right 
  valve 
  there 
  is 
  a 
  prominent, 
  rather 
  stout, 
  elongated 
  posterior 
  

   tooth, 
  the 
  anterior 
  end 
  of 
  which 
  joins 
  closely 
  the 
  cartilage-plate, 
  leav- 
  

   ing 
  scarcely 
  any 
  notch 
  between 
  ; 
  the 
  highest 
  part 
  of 
  the 
  tooth 
  is 
  near 
  

   the 
  middle, 
  the 
  slope, 
  however, 
  is 
  a 
  little 
  steeper 
  anteriorly; 
  a 
  deep 
  

   groove 
  separates 
  the 
  tooth 
  from 
  the 
  thin, 
  slightly 
  excurved 
  dorsal 
  mar- 
  

   gin; 
  anteriorly 
  the 
  margin 
  is 
  but 
  slightly 
  thickened, 
  and 
  shows 
  a 
  very 
  

   narrow, 
  beveled 
  edge 
  externally 
  for 
  the 
  attachment 
  of 
  the 
  thin 
  liga- 
  

   ment 
  ; 
  a 
  similar 
  but 
  more 
  distinct 
  ligamental 
  groove 
  extends 
  from 
  the 
  

   beak 
  to 
  the 
  base 
  of 
  the 
  rostrum; 
  there 
  is 
  a 
  short, 
  rather 
  stout, 
  rib-like 
  

   clavicle 
  or 
  buttress 
  running 
  from 
  beneath 
  the 
  middle 
  of 
  the 
  tooth 
  

   obliquely 
  backward 
  and 
  downward 
  in 
  the 
  direction 
  of 
  the 
  base 
  of 
  the 
  

   rostrum; 
  a 
  less 
  prominent 
  buttress 
  is 
  also 
  present 
  in 
  the 
  left 
  valve. 
  

  

  Length 
  of 
  one 
  of 
  the 
  largest 
  specimens, 
  25 
  mm.; 
  height, 
  15 
  mm.; 
  

   thickness, 
  14 
  mm.; 
  from 
  beak 
  to 
  end 
  of 
  rostrum, 
  13 
  mm.; 
  to 
  antero- 
  

   ventral 
  margin, 
  12 
  mm. 
  One 
  badly 
  broken 
  valve 
  is 
  considerably 
  larger 
  

   than 
  this. 
  There 
  are 
  also 
  two 
  young 
  live 
  specimens 
  which 
  measure 
  

   about 
  G 
  mm. 
  in 
  length 
  and 
  3.5 
  mm. 
  in 
  height. 
  Their 
  form 
  is 
  somewhat 
  

   narrower 
  and 
  longer 
  than 
  in 
  the 
  adult, 
  and 
  the 
  rostrum 
  appears 
  rather 
  

   longer 
  and 
  narrower; 
  the 
  postero-dorsal 
  margin 
  is 
  nearly 
  straight; 
  the 
  

   ventral 
  margin 
  is 
  decidedly 
  concave 
  at 
  the 
  base 
  of 
  the 
  rostrum 
  ; 
  the 
  

   shells 
  are 
  very 
  thin, 
  somewhat 
  transparent 
  and 
  glossy, 
  and 
  have 
  about 
  

   twenty-six 
  sharply 
  defined, 
  considerably 
  elevated,. 
  nearly 
  equal, 
  nar- 
  

   row 
  ribs 
  on 
  the 
  body 
  of 
  the 
  shell, 
  separated 
  by 
  much 
  wider 
  spaces; 
  

   the 
  edge 
  of 
  the 
  left 
  valve 
  overlaps 
  that 
  of 
  the 
  right, 
  especially 
  along 
  

   the 
  base 
  of 
  the 
  rostrum. 
  

  

  In 
  general 
  appearance 
  this 
  species 
  greatly 
  resembles 
  G. 
  multieostata 
  

   Yerrill 
  and 
  Smith. 
  It 
  differs, 
  however, 
  in 
  having 
  a 
  regularly 
  more 
  ovate 
  

   form 
  with 
  the 
  anterior 
  region 
  somewhat 
  narrower 
  and 
  more 
  prolonged 
  

   and 
  the 
  postero- 
  ventral 
  margin 
  less 
  incurved 
  at 
  the 
  base 
  of 
  the 
  ros- 
  

   trum, 
  so 
  that 
  the 
  latter 
  is 
  broader 
  and 
  less 
  differentiated. 
  The 
  exter- 
  

   nal 
  costie 
  differ 
  in 
  being 
  narrow 
  and 
  sharp, 
  separated 
  by 
  broad 
  concave 
  

   interspaces, 
  and 
  of 
  nearly 
  uniform 
  size, 
  there 
  being 
  no 
  marked 
  contrast 
  

   between 
  those 
  on 
  the 
  anterior 
  and 
  posterior 
  portions 
  of 
  the 
  shell, 
  

  

  