﻿72 
  ON 
  THE 
  NAIADES, 
  

  

  Diam. 
  -8, 
  Length 
  -1, 
  Breadth 
  1*9 
  inches. 
  

  

  Shell 
  scaleniform, 
  cuneated, 
  oblique, 
  very 
  inequilateral, 
  angular 
  be- 
  

   hind 
  ; 
  substance 
  of 
  the 
  shell 
  thick 
  before, 
  thinner 
  behind 
  ; 
  beaks 
  ele- 
  

   vated 
  and 
  rounded 
  ; 
  epidermis 
  very 
  finely 
  wrinkled, 
  shining, 
  yellowish 
  

   brown 
  with 
  numerous 
  green 
  flexuous 
  capillary 
  rays 
  over 
  the 
  whole 
  

   disk 
  ; 
  ligament 
  rather 
  short 
  and 
  thick 
  ; 
  cardinal 
  teeth 
  elevated, 
  crenate, 
  

   deeply 
  cleft 
  in 
  the 
  left 
  valve, 
  and 
  emerging 
  from 
  a 
  pit 
  in 
  the 
  right 
  

   valve 
  ; 
  lateral 
  teeth 
  long 
  and 
  nearly 
  straight 
  ; 
  anterior 
  cicatrices 
  distinct 
  ; 
  

   posterior 
  cicatrices 
  nearly 
  distinct; 
  dersal 
  cicatrices 
  situated 
  in 
  the 
  

   centre 
  of 
  the 
  cavity 
  of 
  the 
  beaks 
  ; 
  cavity 
  of 
  the 
  beaks 
  very 
  shallow 
  ; 
  

   nacre 
  beautifully 
  pearly 
  white 
  and 
  iridescent. 
  

  

  Remarks. 
  — 
  I 
  owe 
  to 
  the 
  great 
  kindness 
  of 
  professor 
  Troost 
  the 
  ex- 
  

   amination 
  of 
  his 
  select 
  specimens, 
  which 
  he 
  most 
  obligingly 
  sent 
  to 
  

   me 
  for 
  that 
  purpose. 
  Among 
  them 
  were 
  two 
  specimens 
  of 
  this 
  rare 
  

   and 
  beautiful 
  species, 
  unsurpassed 
  by 
  any 
  other 
  in 
  the 
  delicacy 
  and 
  

   exquisite 
  beauty 
  of 
  its 
  rays. 
  In 
  general 
  form 
  it 
  approaches 
  the 
  scale- 
  

   nius 
  (Rafinesque), 
  but 
  differs 
  from 
  it 
  in 
  the 
  form 
  of 
  the 
  rays 
  altogether. 
  

   It 
  differs 
  also 
  in 
  colour 
  and 
  in 
  having 
  the 
  beaks 
  less 
  retuse. 
  In 
  dedi- 
  

   cating 
  this 
  rare 
  and 
  beautiful 
  species 
  to 
  my 
  friend, 
  professor 
  Troost, 
  I 
  

   do 
  him 
  but 
  an 
  act 
  of 
  simple 
  justice. 
  His 
  constant 
  efforts 
  in 
  the 
  pro- 
  

   motion 
  of 
  the 
  physical 
  sciences 
  are 
  known 
  and 
  acknowledged, 
  and 
  his 
  

   investigation 
  in 
  this 
  branch 
  of 
  conchology 
  will 
  do 
  much 
  to 
  illustrate 
  

   its 
  history 
  in 
  his 
  adopted 
  state. 
  

  

  Unio 
  perdix. 
  Plate 
  XI. 
  fig. 
  31. 
  

  

  Testa 
  ellipticd, 
  postice 
  subangulatd, 
  subsequilaterali. 
  inflatd, 
  transversa 
  ; 
  valvu- 
  

   lis 
  subcrassis 
  ; 
  epidermide 
  luteold, 
  radiis 
  irregulariter 
  interruptis 
  ; 
  dentibus 
  car- 
  

   dinalibus 
  elevatis 
  ; 
  lateralibus 
  prope 
  eorum 
  fines 
  majoribus 
  ; 
  margaritd 
  albd 
  et 
  

   iridescente. 
  

  

  Shell 
  elliptical, 
  subangular 
  behind, 
  nearly 
  equilateral, 
  inflated, 
  transverse 
  ; 
  valves 
  

   rather 
  thick 
  ; 
  epidermis 
  yellowish 
  with 
  irregularly 
  interrupted 
  rays; 
  cardinal 
  teeth 
  

   elevated 
  ; 
  lateral 
  teeth 
  larger 
  near 
  their 
  termination 
  ; 
  nacre 
  pearly 
  white 
  and 
  iri- 
  

   descent. 
  

  

  