﻿890 
  Forty- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  proceeding 
  laterally 
  from 
  the 
  larger 
  ones, 
  precisely 
  as 
  the 
  lateral 
  

   branches 
  proceed 
  from 
  the 
  main 
  stem 
  of 
  Ptilopora. 
  No 
  speci- 
  

   mens 
  have 
  so 
  far 
  been 
  observed 
  showing 
  a 
  gradual 
  change 
  from 
  

   these 
  forms 
  to 
  Ptilopora, 
  but 
  from 
  the 
  facts 
  as 
  shown 
  by 
  other 
  

   forms 
  it 
  is 
  reasonable 
  to 
  infer 
  that 
  such 
  forms 
  have 
  existed 
  

   and 
  will 
  in 
  time 
  be 
  discovered. 
  

  

  The 
  genus 
  Archimedes 
  shows 
  a 
  more 
  radical 
  departure 
  in 
  its 
  

   mode 
  of 
  growth 
  from 
  the 
  ordinary 
  forms 
  of 
  Fenestella. 
  It 
  has 
  

   a 
  spiral 
  mode 
  of 
  growth, 
  the 
  central 
  portion 
  becoming 
  thickened, 
  

   and 
  forming 
  an 
  apparently 
  solid 
  axis, 
  being 
  analogous 
  to 
  the 
  col- 
  

   umella 
  of 
  a 
  univalve 
  shell. 
  

  

  In 
  some 
  species 
  of 
  Fenestellidae 
  from 
  the 
  Corniferous 
  lime- 
  

   stone 
  of 
  the 
  Falls 
  of 
  the 
  Ohio, 
  there 
  is 
  a 
  tendency 
  to 
  a 
  semi-spiral 
  

   growth, 
  and 
  in 
  1883, 
  Prof. 
  E. 
  W. 
  Claypole 
  proposed 
  the 
  name 
  

   Helicopora 
  for 
  a 
  form 
  of 
  the 
  family 
  Fenestellidae 
  in 
  which 
  one 
  

   margin 
  is 
  thickened 
  and 
  contracted 
  in 
  growth 
  by 
  excessive 
  

   secretion 
  and 
  deposition 
  of 
  calcareous 
  matter, 
  leaving 
  the 
  opposite 
  

   margin 
  to 
  increase 
  more 
  rapidly 
  and 
  causing 
  the 
  frond 
  to 
  assume 
  

   the 
  appearance 
  of 
  an 
  incipient 
  spiral 
  growth. 
  

  

  This 
  form 
  is 
  very 
  interesting 
  as 
  it 
  is 
  intermediate 
  between 
  the 
  

   forms 
  mentioned 
  above 
  from 
  the 
  Corniferous 
  limestone 
  and 
  

   Archimedes. 
  

  

  More 
  intermediate 
  forms 
  showing 
  a 
  gradual 
  change 
  will 
  prob- 
  

   ably 
  be 
  discovered. 
  

  

  Remarks 
  on 
  the 
  Various 
  Genera. 
  

  

  The 
  various 
  definitions 
  of 
  the 
  genus 
  Fenestella 
  by 
  Lonsdale 
  in 
  

   1839, 
  the 
  same 
  author 
  in 
  1845 
  and 
  by 
  King 
  in 
  1849 
  have 
  already 
  

   been 
  given 
  in 
  full. 
  

  

  After 
  careful 
  restudy 
  of 
  the 
  subject 
  the 
  following 
  description 
  

   for 
  the 
  genus 
  Fenestella 
  is 
  now 
  offered 
  : 
  

  

  A 
  ramose 
  calcareous 
  bryozoan 
  forming 
  cup-shaped 
  or 
  funnel- 
  

   shaped 
  expansions 
  ; 
  branches 
  bifurcating 
  and 
  connected 
  by 
  

   apparently 
  solid 
  dissepiments; 
  apertures 
  arranged 
  in 
  two 
  par- 
  

   allel 
  rows 
  on 
  one 
  face 
  of 
  the 
  branch, 
  separated 
  by 
  a 
  row 
  of 
  nodes 
  

   or 
  a 
  carina. 
  

  

  The 
  carinae 
  vary 
  greatly 
  in 
  character 
  and 
  appearance, 
  and 
  it 
  

   has 
  been 
  with 
  me 
  a 
  question 
  whether 
  or 
  not 
  such 
  variation 
  is 
  of 
  

   sub-generic 
  importance. 
  

  

  