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  PLATE 
  XX. 
  

  

  Monterey 
  (Middle 
  Miocene) 
  Diatoms. 
  

  

  Fig. 
  la. 
  Actinoptychus 
  undulatus 
  Ehrenberg. 
  

  

  Fig. 
  16. 
  Lithodcsmium 
  cornigerum 
  Brim. 
  

  

  Fig. 
  Ic. 
  Dictyocha 
  gracilis 
  (not 
  a 
  diatom; 
  nature 
  unknown). 
  Enlargement 
  to 
  1.000 
  

  

  diameters 
  of 
  a 
  portion 
  of 
  the 
  slide 
  shown 
  in 
  PL 
  XIX, 
  fig. 
  1. 
  

   Fig. 
  2. 
  Coscinodiscus 
  obscurus 
  A. 
  S., 
  X 
  1.000. 
  From 
  the 
  Monterey 
  shale 
  of 
  theSantaj 
  

  

  Maria 
  district. 
  

   Fig. 
  3. 
  Coscinodiscus 
  subtilis 
  Ehrenberg, 
  X 
  1,000. 
  From 
  the 
  Monterey 
  shale 
  of 
  the 
  

  

  Santa 
  Maria 
  district. 
  

   Fig. 
  4. 
  Coscinodiscus 
  robustus 
  Grew, 
  X 
  1,000. 
  From 
  the 
  Monterey 
  shale 
  of 
  the 
  Santa 
  

  

  Maria 
  district. 
  

  

  142 
  

  

  

  