﻿270 
  G. 
  R. 
  Wieland 
  — 
  On 
  Liassic 
  Floras. 
  

  

  I 
  am 
  disposed 
  to 
  give 
  to 
  every 
  single 
  species 
  and 
  variety 
  of 
  

   cycad 
  foliage, 
  and 
  to 
  every 
  fruit 
  illustrated, 
  a 
  full 
  unit 
  value. 
  

  

  That 
  we 
  shall 
  not 
  be 
  seriously 
  misled 
  in 
  so 
  doing 
  is 
  my 
  con- 
  

   fident 
  belief 
  ; 
  for 
  despite 
  all 
  possible 
  duplication 
  of 
  species 
  due 
  

   to 
  dissociation 
  of 
  fruits 
  from 
  leaves 
  to 
  which 
  they 
  pertained, 
  

   the 
  abundance 
  of 
  cycadoph 
  ytans 
  is 
  clearly 
  extraordinary 
  through- 
  

   out 
  the 
  Mixtecan 
  strata. 
  In 
  fact, 
  without 
  actual 
  count 
  of 
  forms 
  

   directly 
  from 
  the 
  specimens, 
  a 
  matter 
  of 
  considerable 
  difficulty, 
  

   the 
  general 
  impression 
  remains 
  that 
  probably 
  90 
  per 
  cent 
  of 
  

   all 
  the 
  forms 
  collected 
  are 
  leaves 
  or 
  fruits 
  of 
  cycadophytans. 
  

   Whence 
  there 
  can 
  be 
  little 
  doubt 
  that 
  whatever 
  the 
  increase 
  in 
  

   other 
  types 
  as 
  collection 
  goes 
  on, 
  fully 
  fifty 
  valid 
  species 
  of 
  

   cycads 
  will 
  be 
  found 
  in 
  the 
  .Mixtecan 
  horizons, 
  and 
  probably 
  

   many 
  more. 
  On 
  listing 
  the 
  various 
  forms 
  so 
  far 
  definitely 
  

   determined, 
  the 
  following 
  percentages 
  appear 
  : 
  

  

  Table 
  IV. 
  — 
  Rhat-Liassic 
  Flora 
  of 
  Mixteea 
  Alta. 
  

  

  Cycadophytans 
  (10$ 
  Pterophyllums)_ 
  42 
  forms 
  =; 
  70 
  $ 
  

  

  Ferns 
  \ 
  01d 
  ^ 
  e8 
  6 
  " 
  = 
  10 
  

  

  * 
  erns 
  ] 
  Modern 
  types 
  5 
  « 
  =8 
  

  

  Cordaites 
  , 
  _ 
  . 
  5 
  ± 
  " 
  = 
  8 
  

  

  Conifers 
  (Araucarioxy] 
  on) 
  1 
  " 
  = 
  2 
  

  

  Equisetum 
  1 
  " 
  =2 
  

  

  60 
  forms 
  100 
  

  

  In 
  considering 
  Table 
  TV 
  it 
  is 
  to 
  be 
  noted 
  that 
  because 
  of 
  the 
  

   large 
  proportion 
  of 
  cycad 
  fruits 
  and 
  a 
  relatively 
  poor 
  conser- 
  

   vation 
  of 
  ferns 
  in 
  most 
  strata 
  of 
  the 
  Consuelo 
  section, 
  already 
  

   commented 
  on, 
  the 
  fern 
  element 
  appears 
  unduly 
  small. 
  There 
  

   is 
  probably, 
  therefore, 
  no 
  very 
  strongly 
  marked 
  departure 
  from 
  

   the 
  usual 
  Liassic 
  fern 
  proportion 
  of 
  about 
  one-third 
  of 
  all 
  

   the 
  plants 
  recovered. 
  But 
  the 
  drop 
  from 
  fully, 
  or 
  over, 
  

   one-half 
  of 
  all 
  plants 
  in 
  the 
  Rhatic 
  is 
  none 
  the 
  less 
  striking 
  ; 
  

   and 
  although 
  the 
  ferns 
  again 
  seem 
  to 
  reach 
  large 
  relative 
  num- 
  

   bers 
  with 
  the 
  accession 
  of 
  numerous 
  recent 
  types 
  in 
  the 
  late 
  

   Jurassic 
  and 
  Wealden, 
  the 
  Liassic 
  displacement 
  of 
  ferns 
  and 
  

   dominance 
  of 
  cycadophytans 
  is 
  inescapably 
  clear. 
  These 
  rela- 
  

   tions 
  at 
  once 
  appear 
  from 
  the 
  summary 
  (Table 
  TV 
  a) 
  of 
  the 
  

   Rhatic 
  flora 
  of 
  Tonkin, 
  as 
  so 
  fully 
  and 
  thoroughly 
  elaborated 
  

   by 
  Zeilier. 
  

  

  Table 
  IVa.—Rhcetic 
  Flora 
  of 
  Tonkin. 
  

  

  Ferns 
  (mainly 
  older 
  types) 
  _ 
  _ 
  _ 
  . 
  26 
  species 
  = 
  48 
  $ 
  

  

  Cycads 
  (largely 
  Pterophyllums) 
  18 
  " 
  — 
  33-5 
  

  

  Conifers 
  (Ex. 
  JVoeggerathiopsis= 
  Cordaites) 
  5 
  " 
  = 
  9 
  

  

  Equisetums 
  — 
  3 
  " 
  = 
  5*5 
  

  

  Ginkgo 
  _ 
  1 
  " 
  = 
  2 
  

  

  54 
  species 
  

  

  