﻿NO. 
  1139. 
  DEEP-WATER 
  MOLLUSCA—VEBRILL 
  AND 
  BUSH. 
  813 
  

  

  Family 
  POROMYID^. 
  

  

  POROMYA 
  SUBLEVIS 
  VerriU, 
  variety 
  MICRODONTA 
  Dall. 
  

   (Plates 
  LXXVI, 
  figs. 
  1, 
  2; 
  LXXXVII, 
  fig. 
  1.) 
  

  

  Poromya 
  sitblevia 
  Yekrill, 
  Trans. 
  Conn. 
  Acad., 
  YI, 
  pp. 
  221, 
  277, 
  pi. 
  xxxii, 
  fig, 
  

   21, 
  1884; 
  Expl. 
  Albatross, 
  Report 
  U. 
  S. 
  Com. 
  Fish 
  and 
  Fisheries 
  for 
  1883, 
  

   p. 
  574, 
  pi. 
  XXX, 
  fig. 
  128, 
  1885.— 
  Dall, 
  Bull. 
  Mus. 
  Comp. 
  Zo51., 
  XII, 
  pp. 
  281, 
  

   282, 
  1886; 
  XYIII, 
  p. 
  448, 
  1889 
  (variety?); 
  Bull. 
  U. 
  S. 
  Nat. 
  Mus., 
  No. 
  37, 
  p. 
  

   68, 
  pi. 
  Lxv, 
  fig. 
  128, 
  1889. 
  

  

  Poromya 
  microdonta 
  Dall, 
  Proc. 
  U. 
  S.-Nat. 
  Mus., 
  XII, 
  p. 
  290, 
  pi. 
  viii, 
  fig. 
  6, 
  1889 
  

   (variety?). 
  

  

  Shell 
  rather 
  large, 
  thick, 
  well-rounded, 
  cordate, 
  iuequivalved, 
  very- 
  

   tumid, 
  with 
  very 
  large, 
  prominent 
  umbos 
  which 
  are 
  strongly 
  curved 
  

   forward 
  spirally; 
  beaks 
  large; 
  luuule 
  small, 
  cordate, 
  often 
  not 
  very 
  

   distinct. 
  The 
  shell 
  varies 
  considerably 
  in 
  outline 
  and 
  size 
  and 
  eleva- 
  

   tion 
  of 
  the 
  umbos; 
  in 
  most 
  specimens 
  the 
  height 
  equals 
  or 
  slightly 
  

   exceeds 
  the 
  length 
  ; 
  the 
  outline 
  of 
  the 
  cavity 
  of 
  the 
  shell 
  is 
  usually 
  

   somewhat 
  elliptical, 
  the 
  length 
  decidedly 
  exceeding 
  the 
  height, 
  but 
  

   sometimes 
  it 
  is 
  nearly 
  circular. 
  The 
  anterior 
  and 
  posterior 
  margins 
  are 
  

   usually 
  pretty 
  evenly 
  rounded; 
  the 
  ventral 
  margin 
  usually 
  projects 
  a 
  

   little 
  in 
  the 
  middle; 
  the 
  beak 
  is 
  situated 
  in 
  front 
  of 
  the 
  median 
  line. 
  

   Externally 
  the 
  shell 
  is 
  nearly 
  smooth 
  and 
  is 
  covered 
  with 
  a 
  thin, 
  

   closely 
  adherent, 
  brownish-yellow 
  epidermis; 
  under 
  a 
  lens 
  the 
  sur- 
  

   face 
  shows 
  minute 
  raised 
  points 
  or 
  granules 
  which 
  are 
  arranged 
  in 
  

   radial 
  rows 
  that 
  become 
  more 
  distinct 
  and 
  crowded 
  posteriorly 
  but 
  

   for 
  the 
  most 
  part 
  disappear 
  on 
  the 
  most 
  ijrominent 
  part 
  of 
  the 
  umbos. 
  

   These 
  granule 
  like 
  points 
  are 
  variable 
  in 
  number 
  and 
  distinctness, 
  in 
  

   some 
  specimens 
  being 
  nearly 
  obsolete 
  and 
  in 
  others 
  distinct 
  aud 
  regu- 
  

   hirly 
  arranged; 
  the 
  epidermis 
  often 
  also 
  shows 
  fine 
  lines 
  of 
  growth; 
  

   the 
  beaks 
  are 
  smooth 
  and 
  shining. 
  The 
  left 
  valve 
  has 
  a 
  p'isterior, 
  

   wave-like, 
  radial 
  depression, 
  and 
  behind 
  this 
  a 
  low, 
  rounded 
  ridge 
  

   projecting 
  at 
  the 
  margin 
  as 
  a 
  slight 
  siphonal 
  lobe; 
  in 
  the 
  right 
  valve, 
  

   the 
  corresponding 
  lobe 
  and 
  depression 
  are 
  only 
  faintly 
  marked 
  in 
  most 
  

   cases. 
  The 
  right 
  valve 
  is 
  larger 
  than 
  the 
  left 
  and 
  overlaps 
  it 
  consider- 
  

   ably 
  along 
  the 
  ventral 
  margin 
  and 
  both 
  in 
  front 
  of 
  and 
  behind 
  the 
  

   beaks.' 
  The 
  interior 
  is 
  pearly 
  and 
  often 
  shows 
  radial 
  striations. 
  Tlie 
  

   hinge-margin 
  is 
  considerably 
  thickened 
  and 
  strongly 
  curved; 
  the 
  right 
  

   valve 
  has 
  a 
  large, 
  thick, 
  somewhat 
  rounded 
  tooth 
  just 
  beneath 
  the 
  

   beak 
  and 
  adnate 
  to 
  the 
  inner 
  surface 
  of 
  the 
  shell, 
  for 
  some 
  distance 
  

   within 
  the 
  cavity 
  of 
  the 
  beak 
  and 
  to 
  the 
  thickened 
  edge 
  behind 
  the 
  

   beak, 
  but 
  separated 
  from 
  the 
  anterior 
  margin 
  by 
  a 
  deep, 
  curved 
  furrow 
  

   in 
  the 
  lunular 
  area 
  ; 
  the 
  lunular 
  margin 
  is 
  convex 
  and 
  somewhat 
  everted, 
  

   separated 
  from 
  the 
  rest 
  of 
  the 
  anterior 
  margin 
  by 
  a 
  slight 
  notch. 
  The 
  

   ligament 
  is 
  rather 
  long 
  and 
  well-rounded 
  and 
  its 
  groove 
  extends 
  for- 
  

   ward 
  in 
  a 
  curved 
  furrow 
  under 
  the 
  beak 
  ; 
  it 
  extends 
  backward 
  in 
  a 
  

   curved 
  line 
  parallel 
  with 
  the 
  margin 
  of 
  the 
  shell 
  for 
  some 
  distance 
  

  

  