﻿AND 
  OTHER 
  FAMILIES. 
  71 
  

  

  are 
  almost 
  flat. 
  The 
  deposit 
  of 
  the 
  nacre 
  after 
  this 
  forms 
  an 
  angle 
  of 
  

   nearly 
  45° 
  with 
  the 
  surface 
  which 
  it 
  has 
  left, 
  thus 
  forming 
  a 
  hump, 
  or 
  

   obtuse 
  angle 
  point, 
  directly 
  on 
  the 
  umbo. 
  This 
  causes 
  the 
  curious 
  

   result, 
  that 
  when 
  the 
  shell 
  is 
  from 
  one 
  third 
  to 
  three 
  fourths 
  grown, 
  

   it 
  will 
  rest, 
  when 
  so 
  placed, 
  on 
  the 
  portion 
  of 
  surface 
  between 
  the 
  

   point 
  of 
  the 
  beak 
  and 
  the 
  umbo, 
  the 
  basal 
  margin 
  remaining 
  in 
  the 
  air. 
  

   In 
  its 
  general 
  characters 
  it 
  resembles 
  the 
  irroratus, 
  but 
  may 
  at 
  once 
  be 
  

   distinguished 
  by 
  the 
  hump. 
  It 
  is 
  devoid 
  of 
  tubercles, 
  while 
  the 
  irro- 
  

   ratus 
  is 
  sometimes 
  covered 
  with 
  them, 
  particularly 
  on 
  the 
  posterior 
  

   part. 
  It 
  differs 
  somewhat 
  also 
  in 
  the 
  rays, 
  the 
  spots 
  in 
  those 
  of 
  the 
  

   dromas 
  being 
  larger, 
  and 
  generally 
  better 
  defined. 
  The 
  outline 
  differs 
  

   in 
  being 
  less 
  elongated, 
  being 
  disposed 
  to 
  be 
  more 
  oblique 
  or 
  more 
  

   transverse. 
  In 
  regard 
  to 
  the 
  structure 
  of 
  the 
  animal, 
  I 
  am 
  not 
  prepared 
  

   to 
  say 
  that 
  it 
  differs 
  from 
  that 
  of 
  the 
  irroratus.* 
  Not 
  having 
  had 
  an 
  

   opportunity 
  to 
  examine 
  the 
  animal, 
  I 
  can 
  only 
  judge 
  by 
  analogy, 
  which 
  

   would, 
  I 
  think, 
  induce 
  one 
  to 
  conclude 
  that 
  the 
  same 
  curious 
  pendent 
  

   oviducts 
  would 
  be 
  found 
  in 
  both. 
  I 
  hope 
  to 
  be 
  able 
  to 
  procure 
  from 
  

   professor 
  Troost 
  a 
  specimen 
  in 
  that 
  period 
  of 
  gestation. 
  

  

  Unio 
  Troostensis. 
  Plate 
  X. 
  fig. 
  30. 
  

  

  Testa 
  scalend, 
  cuneatd, 
  obliqud, 
  valde 
  insequilaterali 
  ; 
  valvidis 
  antice 
  crassiori- 
  

   bus; 
  natibus 
  subterminalibus 
  ; 
  epidermide 
  luteold, 
  radiis 
  capillaribus 
  multis 
  ; 
  den- 
  

   tibus 
  cardinalibus 
  elevatis, 
  cristatis 
  ; 
  lateralibus 
  subrectis 
  ; 
  margaritd 
  albd 
  et 
  iri- 
  

   descente. 
  

  

  Shell 
  scaleniform, 
  wedge 
  shaped, 
  oblique, 
  very 
  inequilateral 
  ; 
  valves 
  thicker 
  ante- 
  

   riorly 
  ; 
  beaks 
  nearly 
  terminal 
  ; 
  epidermis 
  yellowish, 
  rilled 
  with 
  numerous 
  capillary 
  

   rays 
  ; 
  cardinal 
  teeth 
  elevated, 
  crested 
  ; 
  lateral 
  teeth 
  nearly 
  straight 
  ; 
  nacre 
  pearly 
  white 
  

   and 
  iridescent. 
  

  

  Hab. 
  Cumberland 
  River. 
  Professor 
  Troost. 
  

  

  My 
  Cabinet. 
  

   Cabinet 
  of 
  Professor 
  Troost. 
  

  

  * 
  See 
  vol. 
  iii. 
  p. 
  271. 
  

  

  