﻿North 
  American 
  Diatomaceae 
  71 
  

  

  of 
  the 
  fin-like 
  blade, 
  acting 
  as 
  an 
  anchor, 
  serves 
  a 
  similar 
  purpose, 
  is 
  

   a 
  question. 
  

  

  Abnormal 
  form 
  of 
  Aulacodiscus 
  oregonus 
  Harv. 
  & 
  Bail. 
  

  

  In 
  an 
  article 
  entitled, 
  '' 
  Atdacodiscus 
  oregonus 
  with 
  two 
  cen- 
  

   tres," 
  Dr. 
  Christopher 
  Johnston* 
  describes 
  and 
  illustrates 
  a 
  form 
  

   which 
  he 
  considers 
  "as 
  an 
  evidence 
  of 
  the 
  action 
  of 
  an 
  excessive 
  

   developmental 
  potency." 
  In 
  normal 
  forms 
  of 
  A, 
  oregonus 
  the 
  

   processes, 
  although 
  variable 
  in 
  number, 
  are 
  symmetrically 
  ar- 
  

   ranged 
  near 
  the 
  border. 
  In 
  a 
  specimen 
  discovered 
  by 
  Mr. 
  

   William 
  C. 
  Stevenson, 
  Jr., 
  of 
  Philadelphia, 
  in 
  recent 
  material 
  

   from 
  Monterey 
  Bay, 
  California, 
  the 
  valve 
  is 
  divided, 
  somewhat 
  

   asymmetrically, 
  into 
  six 
  parts, 
  each 
  of 
  which 
  contains 
  a 
  hyaline 
  

   centre 
  and 
  processes 
  with 
  more 
  or 
  less 
  evident 
  furrows. 
  The 
  

   form 
  may 
  be 
  abnormal 
  but 
  Mr. 
  H. 
  C, 
  Wheeler, 
  of 
  Montreal, 
  has 
  

   sent 
  me 
  two 
  specimens 
  mounted 
  from 
  material 
  from 
  Port 
  Town- 
  

   send, 
  Washington, 
  one 
  of 
  which 
  somewhat 
  resembles 
  Mr. 
  Steven- 
  

   son's 
  form, 
  which 
  is 
  illustrated 
  in 
  FiG. 
  5, 
  while 
  the 
  other 
  is 
  sym- 
  

  

  me 
  

  

  Joh 
  

  

  ton*s 
  form. 
  Whether 
  these 
  specimens 
  are 
  evidences 
  of 
  the 
  for- 
  

   mation 
  of 
  gonidia 
  may 
  be 
  a 
  question, 
  but 
  they 
  appear 
  to 
  be 
  too 
  

   I 
  numerous 
  to 
  be 
  considered 
  as 
  mere 
  abnormalities. 
  It 
  is 
  to 
  be 
  

   noted 
  that 
  numerous 
  valves 
  occur 
  in 
  rich 
  gatherings 
  much 
  smaller 
  

   than 
  the 
  normal 
  valve 
  and 
  of 
  uniform 
  size, 
  equal 
  to 
  that 
  of 
  the 
  

   partially 
  formed 
  valves 
  in 
  the 
  specimen 
  figured. 
  In 
  each 
  of 
  the 
  

   specimens 
  examined 
  the 
  internal 
  finely 
  granular 
  plate 
  described 
  

   by 
  Mr. 
  R. 
  C. 
  Greenleaff 
  is 
  distinctly 
  shown, 
  but 
  its 
  function 
  in 
  

   the 
  formation 
  of 
  new 
  valves 
  is 
  problematical. 
  

  

  * 
  Am. 
  Jour. 
  Micros, 
  i: 
  82. 
  1876. 
  

   t 
  Mo. 
  Micros. 
  Jour. 
  2 
  : 
  326. 
  1869- 
  

  

  