﻿art. 
  21 
  CYCLOSTOMATOUS 
  BRYOZOA 
  CANU 
  AND 
  BASSLER 
  87 
  

  

  Genotype. 
  — 
  Siphodictyum 
  gracile 
  Lonsdale, 
  1849. 
  Cretaceous 
  (Ap- 
  

   tian, 
  Albian 
  and 
  Campanian). 
  

  

  Affinities. 
  — 
  Gregory, 
  1S99, 
  classified 
  this 
  genus 
  in 
  the 
  Horneridae. 
  

   This 
  is 
  correct 
  so 
  far 
  as 
  the 
  exterior 
  aspect 
  and 
  the 
  nature 
  of 
  the 
  

   tubes 
  is 
  concerned, 
  but 
  the 
  ovicell 
  is 
  quite 
  different 
  and 
  is 
  of 
  the 
  

   type 
  characteristic 
  of 
  the 
  Ascosocciidae. 
  

  

  The 
  genus 
  differs 
  for 
  Laterocavea 
  D'Orbigny, 
  1853, 
  in 
  the 
  nature 
  of 
  

   the 
  adventitious 
  pores, 
  which 
  are 
  vacuoles 
  and 
  not 
  mesopores. 
  In 
  

   the 
  occurrence 
  of 
  large 
  noncellular 
  areas 
  the 
  two 
  genera 
  are 
  very 
  

   close 
  but 
  the 
  areas 
  are 
  arranged 
  quite 
  differently. 
  

  

  Siphodictyum 
  differs 
  from 
  Reteporidea 
  D'Orbigny, 
  1853, 
  in 
  the 
  

   nature 
  of 
  the 
  adventitious 
  pores, 
  which 
  are 
  vacuoles 
  placed 
  at 
  the 
  

   bottom 
  of 
  the 
  sulci, 
  and 
  in 
  its 
  nonreticulate 
  zoarium. 
  

  

  The 
  physiological 
  function 
  of 
  the 
  vacuoles 
  in 
  recent 
  Homera 
  is 
  

   not 
  known, 
  and 
  we 
  are 
  unable, 
  therefore, 
  to 
  understand 
  the 
  impor- 
  

   tance 
  of 
  the 
  large 
  noncellular 
  spaces 
  on 
  species 
  of 
  Laterocavea 
  and 
  

   Siphodictyum. 
  

  

  Certain 
  branches 
  are 
  petaloporoid, 
  as 
  they 
  have 
  orifices 
  entirely 
  

   around 
  the 
  zoarium. 
  

  

  SIPHODICTYUM 
  GRACILE, 
  Lonsdale, 
  1849 
  

  

  Plate 
  14, 
  figs. 
  14-21 
  

  

  1899. 
  Siphodictyum 
  gracile 
  Gregory, 
  Catalogue 
  of 
  the 
  Cretaceous 
  Bryozoa 
  in 
  

   the 
  British 
  Museum, 
  vol. 
  1, 
  p. 
  363, 
  fig. 
  45, 
  pi. 
  12, 
  figs. 
  14, 
  15. 
  (Bibliog- 
  

   raphy.) 
  

  

  Measurements. 
  — 
  Diameter 
  of 
  orifices, 
  0.08 
  mm.; 
  diameter 
  of 
  zoo- 
  

   ecia, 
  0.16 
  mm.; 
  distance 
  of 
  orifices 
  along 
  margin, 
  0.40 
  mm.; 
  diam- 
  

   eter 
  of 
  large 
  branches, 
  .070 
  mm. 
  

  

  Structure. 
  — 
  The 
  zoarium 
  is 
  cylindrical, 
  dichotomously 
  branched. 
  

   The 
  peristomes 
  are 
  salient, 
  adjacent, 
  and 
  arranged 
  in 
  transverse 
  

   rows 
  interrupted 
  in 
  the 
  middle, 
  on 
  the 
  anterior 
  face 
  of 
  the 
  colony. 
  

   They 
  are 
  separated 
  in 
  the 
  longitudinal 
  direction 
  by 
  4 
  to 
  6 
  vacuoles. 
  

   On 
  the 
  posterior 
  face 
  only 
  vacuoles 
  occur, 
  at 
  the 
  base 
  of 
  broad 
  sulci 
  

   of 
  little 
  depth; 
  the 
  sulci 
  are 
  rarely 
  longitudinal 
  and 
  are 
  almost 
  always 
  

   more 
  or 
  less 
  oblique. 
  This 
  arrangement 
  prevents 
  the 
  discovery 
  of 
  

   the 
  true 
  nature 
  of 
  the 
  pores 
  as 
  observed 
  in 
  thin 
  sections. 
  The 
  dorsal 
  

   is 
  sometimes 
  much 
  reduced. 
  

  

  In 
  longitudinal 
  section 
  the 
  zoarium 
  appears 
  surrounded 
  by 
  a 
  very 
  

   thick 
  lamellar 
  epitheca; 
  the 
  tubes 
  are 
  short, 
  cylindrical, 
  with 
  peri- 
  

   stome, 
  recurved 
  at 
  their 
  extremity, 
  with 
  triparietal 
  gemmation, 
  and 
  

   arranged 
  around 
  a 
  central 
  axis. 
  The 
  tubes 
  oriented 
  toward 
  the 
  

   frontal 
  are 
  complete 
  and 
  separated 
  in 
  the 
  recurved 
  portions 
  by 
  

   the 
  epitheca 
  perforated 
  by 
  vacuoles; 
  the 
  tubes 
  oriented 
  toward 
  the 
  

  

  