﻿PLATE 
  IV. 
  

  

  Dignomia 
  alveata, 
  Hall. 
  

  

  Fig. 
  1. 
  — 
  A 
  specimen 
  showing 
  both 
  valves, 
  but 
  very 
  poorly 
  litho- 
  

   graphed. 
  The 
  purpose 
  of 
  this 
  illustration 
  was 
  to 
  elucidate 
  a 
  structural 
  

   feature 
  in 
  this 
  genus 
  which 
  has 
  not 
  before 
  been 
  well 
  understood. 
  In 
  

   the 
  specimen 
  the 
  valve 
  at 
  the 
  left 
  lies 
  beneath 
  the 
  other 
  and 
  has 
  been 
  

   exposed 
  by 
  the 
  breaking 
  away 
  of 
  the 
  filling 
  of 
  the 
  latter. 
  The 
  valve 
  

   at 
  the 
  right 
  is 
  evidently, 
  from 
  the 
  greater 
  prolongation 
  of 
  the 
  apex 
  

   (not 
  shown 
  in 
  the 
  figure), 
  the 
  pedicle-valve, 
  and 
  its 
  internal 
  ridges 
  

   have 
  been 
  exposed 
  by 
  the 
  removal, 
  through 
  weathering, 
  of 
  the 
  prin- 
  

   cipal 
  part 
  of 
  the 
  substance 
  of 
  the 
  valve. 
  In 
  the 
  other 
  valve 
  the 
  internal 
  

   surface 
  is 
  exposed. 
  The 
  specimen 
  shows 
  that 
  the 
  diverging 
  umbonal 
  

   ridges 
  are 
  about 
  equally 
  developed 
  in 
  both 
  valves 
  and 
  when 
  the 
  valves 
  

   were 
  in 
  their 
  normal 
  position 
  the 
  ridges 
  of 
  one 
  must 
  have 
  been 
  situated 
  

   within 
  those 
  of 
  the 
  other, 
  and 
  thus 
  probably 
  have 
  formed 
  a 
  rudi- 
  

   mentary 
  articulation. 
  The 
  median 
  septa 
  of 
  the 
  two 
  valves 
  differ 
  and 
  

   afford 
  a 
  means 
  of 
  distinguishing 
  the 
  two 
  when 
  found 
  separately. 
  

   That 
  of 
  the 
  brachial 
  valve 
  is 
  a 
  sharply 
  elevated 
  and 
  narrow 
  ridge, 
  

   while 
  that 
  of 
  the 
  opposite 
  valve 
  is 
  broad, 
  flat 
  and 
  scarcely 
  elevated. 
  

  

  Upper 
  Hamilton 
  shales, 
  between 
  225 
  and 
  350 
  feet 
  in 
  the 
  Livonia 
  

   shaft. 
  

  

  Orthothetes 
  bellulus, 
  sp. 
  nov. 
  

  

  Fig. 
  2. 
  — 
  An 
  internal 
  cast 
  of 
  the 
  brachial- 
  valve. 
  

   Fig. 
  3. 
  — 
  The 
  pedicle-valve. 
  

  

  Fig. 
  4. 
  — 
  An 
  enlargement 
  of 
  portion 
  of 
  the 
  surface. 
  

   Lower 
  limestone 
  in 
  the 
  Marcellus 
  shales, 
  at 
  854 
  feet 
  in 
  the 
  Livonia 
  

   shaft. 
  

  

  Chonostrophia 
  reversa, 
  Whitfield 
  (sp.). 
  

  

  Fig. 
  5. 
  — 
  Exterior 
  of 
  a 
  pedicle- 
  valve 
  with 
  convex 
  umbo 
  and 
  flat 
  or 
  

   slightly 
  concave 
  anterior 
  surface. 
  X 
  3 
  . 
  

  

  Gorniferous 
  limestone, 
  Union 
  Springs, 
  JV. 
  Y. 
  

  

  381 
  

  

  