﻿The 
  Fauna 
  of 
  the 
  Niagara 
  Group. 
  149 
  

  

  valve 
  closely 
  covered 
  with 
  minute 
  setiform 
  spines, 
  directed 
  

   from 
  the 
  apex 
  toward 
  the 
  margin. 
  Yentral 
  or 
  lower 
  valve 
  

   with 
  the 
  exterior 
  portion 
  merely 
  a 
  calcareous 
  ring, 
  and 
  the 
  

   central 
  area 
  without 
  any 
  decided 
  character. 
  

  

  When 
  the 
  setae 
  are 
  removed, 
  the 
  surface 
  appears 
  strongly 
  

   punctate 
  — 
  a 
  character 
  which 
  distinguishes 
  this 
  form 
  from 
  C. 
  

   Siluriana. 
  

  

  PHOLIDOPS 
  Hall. 
  

   Pholidops 
  ovalis. 
  

  

  Plate 
  u 
  21, 
  Figs 
  1 
  and 
  2. 
  

  

  Pholidops 
  ovalis 
  Hall. 
  Traus. 
  Alb. 
  lust., 
  vol. 
  iv, 
  p. 
  209. 
  Abstract 
  p. 
  15 
  ; 
  May, 
  

   1863. 
  

  

  Shell 
  round-oval, 
  somewhat 
  broader 
  anterior 
  to 
  the 
  middle 
  ; 
  

   valves 
  equally 
  convex 
  ; 
  apex 
  situated 
  about 
  one-third 
  the 
  

   length 
  of 
  the 
  shell 
  from 
  the 
  posterior 
  end. 
  Surface 
  marked 
  

   by 
  concentric 
  lines 
  of 
  growth, 
  which 
  are 
  strongly 
  lamellose 
  

   toward 
  the 
  margin. 
  

  

  This 
  species 
  diners 
  from 
  P. 
  (Orbicula) 
  squamiformis 
  of 
  the 
  

   Niagara 
  group 
  of 
  New 
  York, 
  in 
  the 
  greater 
  convexity, 
  more 
  

   closely 
  arranged 
  concentric 
  lines 
  of 
  growth, 
  and 
  in 
  being 
  

   more 
  regularly 
  and 
  broadly 
  oval. 
  Rare. 
  

  

  ORTHIS 
  Dalman. 
  

   Okthis 
  hybrida 
  Sowerby. 
  

  

  Plate 
  21, 
  Figs 
  18-25. 
  

  

  Orthis 
  hybrida 
  Sow. 
  Muech. 
  Sil. 
  Syst., 
  p.:630, 
  pi. 
  13, 
  fig 
  11. 
  1839. 
  

   Orthis 
  hybrida 
  Sow. 
  Hall. 
  Pal. 
  N. 
  Y., 
  vol. 
  ii, 
  p. 
  253, 
  pi. 
  52, 
  figs. 
  4a-4o. 
  1852. 
  

   Orthis 
  hybrida? 
  Sow. 
  Hall* 
  Traus. 
  Alb. 
  Inst., 
  vol. 
  iv, 
  p. 
  209. 
  Abstract 
  p. 
  15; 
  

   May, 
  1863. 
  

  

  This 
  species 
  is 
  very 
  common 
  in 
  the 
  locality. 
  The 
  numerous 
  

   specimens 
  examined 
  can 
  be 
  easily 
  separated 
  into 
  two 
  distinct 
  

   varieties, 
  one 
  of 
  them 
  narrow 
  and 
  ventricose, 
  with 
  the 
  an- 
  

   terior 
  margins 
  thickened 
  and 
  strongly 
  lamellose, 
  presenting 
  

   strong 
  varices 
  of 
  growth 
  ; 
  another, 
  broader 
  and 
  less 
  ventri- 
  

   cose, 
  with 
  sometimes 
  a 
  wide 
  sinuosity 
  in 
  front. 
  Few 
  of 
  the 
  

   specimens 
  have 
  the 
  broad 
  sinuosity 
  of 
  the 
  ventral 
  valve 
  and 
  

   ventricose 
  upper 
  portion 
  of 
  the 
  shell, 
  so 
  characteristic 
  of 
  the 
  

   New 
  York 
  species 
  which 
  has 
  been 
  referred 
  to 
  0. 
  hybrida. 
  

  

  