﻿DEEP-WATER 
  MOLLUSC 
  A— 
  VERRILL 
  AND 
  HUSH. 
  777 
  

  

  Family 
  PHOLADID^E. 
  

  

  MARTESIELLA, 
  new 
  subgenus. 
  

  

  This 
  subgeueric 
  name 
  is 
  proposed 
  for 
  the 
  following 
  species, 
  whi(3h 
  

   differs 
  from 
  Martesia 
  in 
  having 
  a 
  well 
  detiued, 
  elongated, 
  median, 
  dorsal 
  

   l)late, 
  posterior 
  to 
  the 
  umbos, 
  in 
  addition 
  to 
  the 
  shield-shaped 
  one 
  over 
  

   them. 
  

  

  MARTESIA 
  (MARTESIELLA) 
  FRAGILIS, 
  new 
  species. 
  

   (Plate 
  LXXIX, 
  fig. 
  10.) 
  

  

  Shell 
  small, 
  white, 
  thin, 
  fragile, 
  wedge-shaped. 
  The 
  anterior 
  end 
  is 
  

   very 
  short 
  and 
  broadly 
  rounded, 
  the 
  aperture 
  nearly 
  closed 
  in 
  our 
  

   largest 
  specimen 
  by 
  a 
  i)air 
  of 
  callous 
  plates. 
  The 
  antero 
  dorsal 
  margin 
  is 
  

   recurved 
  toward 
  the 
  umbos, 
  but 
  not 
  appressed, 
  and 
  forms 
  a 
  deep, 
  spiral, 
  

   open 
  cavity. 
  The 
  valves 
  have 
  a 
  very 
  obtuse 
  anterior 
  emargination. 
  

   A 
  broad 
  and 
  moderately 
  deep 
  sulcus 
  runs 
  from 
  the 
  beak 
  to 
  the 
  ventral 
  

   margin; 
  in 
  front 
  of 
  this 
  the 
  surface 
  is 
  covered 
  by 
  thin 
  concentric 
  ribs, 
  

   which 
  curve 
  downwai 
  d 
  at 
  the 
  sulcus 
  and 
  form 
  a 
  distinct 
  angle 
  in 
  line 
  

   with 
  the 
  anterior 
  emargination 
  and 
  corresponding 
  with 
  a 
  slight 
  ridge 
  

   on 
  the 
  surface: 
  these 
  concentric 
  ribs 
  are 
  crossed 
  by 
  fine 
  radiating 
  lines, 
  

   which 
  produce 
  tine 
  serrations 
  on 
  their 
  edges. 
  Posterior 
  to 
  the 
  sulcus 
  

   the 
  surface 
  is 
  marked 
  only 
  by 
  irregular 
  lines 
  of 
  growth, 
  which, 
  near 
  it, 
  

   take 
  the 
  form 
  of 
  more 
  distinct 
  grooves 
  or 
  ridges. 
  The 
  posterior 
  end 
  is 
  

   prolonged, 
  conipressed, 
  and 
  bluntly 
  rounded. 
  The 
  umbonal 
  plate 
  is 
  

   thick, 
  relatively 
  large, 
  and 
  usually 
  heart-shaped, 
  with 
  the])osteri()r 
  end 
  

   broader 
  and 
  distinctly 
  emarginate 
  in 
  the 
  middle; 
  the 
  anterior 
  end 
  

   tapers 
  somewhat 
  and 
  is 
  blunt 
  and 
  angulated, 
  or 
  sometimes 
  subacute. 
  

   The 
  posterior 
  dorsal 
  plate 
  is 
  long, 
  narrow, 
  and 
  somewhat 
  siiatulate 
  or 
  

   clavate, 
  and 
  stands 
  well 
  in 
  relief 
  above 
  the 
  dorsal 
  margin, 
  with 
  the 
  

   edges 
  free 
  and 
  the 
  narrow 
  anterior 
  end 
  running 
  under 
  the 
  ])osterior 
  

   end 
  of 
  the 
  umbonal 
  plate. 
  

  

  Length 
  of 
  one 
  of 
  the 
  largest 
  specimens, 
  7 
  mm.; 
  height, 
  4..") 
  mm. 
  r 
  

   thickness, 
  4 
  mm. 
  

  

  Young 
  specimens 
  3 
  or 
  4 
  mm. 
  in 
  length 
  are 
  relatively 
  shorter 
  and 
  

   thicker 
  than 
  the 
  larger 
  ones, 
  but 
  even 
  these 
  have 
  the 
  anterior 
  callous 
  

   pretty 
  well 
  developed; 
  the 
  umbonal 
  plate 
  is 
  usually 
  shield-shaped, 
  

   the 
  lateral 
  borders 
  emarginate, 
  in 
  contact 
  with 
  the 
  most 
  prominent 
  

   part 
  of 
  the 
  umbos; 
  the 
  posterior 
  border 
  is 
  distinctly 
  emarginate, 
  and 
  

   the 
  anterior 
  end 
  has 
  a 
  central 
  point 
  or 
  mucro, 
  sometimes 
  detined 
  by 
  

   slightly 
  concave 
  posterior 
  edges. 
  

  

  Many 
  live 
  specimens 
  were 
  found 
  in 
  a 
  piece 
  of 
  wood 
  Hoating 
  near 
  

   station 
  2566, 
  N. 
  lat. 
  37^ 
  23', 
  W. 
  long. 
  QS<^ 
  8', 
  1885. 
  

  

  