﻿1002 
  Forty- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  layer 
  of 
  the 
  shell 
  is 
  usually, 
  probably 
  always, 
  covered 
  with 
  

   numerous 
  fine, 
  short 
  spinules 
  ; 
  these, 
  when 
  removed, 
  leave 
  the 
  

   surface 
  with 
  only 
  regularly 
  concentric 
  growth-lines, 
  marked 
  by 
  

   papillae 
  which 
  are 
  the 
  bases 
  of 
  the 
  spinules. 
  

  

  Type, 
  Spirt 
  f 
  era 
  ventricosa, 
  Hall. 
  Lower 
  Helderberg 
  group. 
  

  

  Distribution. 
  Upper 
  Silurian 
  — 
  Lower 
  Carboniferous. 
  

  

  Cyclospira, 
  Hall. 
  1893. 
  

   (Plate 
  40, 
  figs. 
  1-3.) 
  

  

  The 
  type 
  of 
  this 
  genus, 
  Atrypa 
  bisulcata, 
  is 
  a 
  sub 
  trihedral 
  

   shell 
  with 
  a 
  very 
  convex 
  pedicle-valve 
  and 
  a 
  depressed 
  brachial 
  

   valve. 
  The 
  larger 
  valve 
  has 
  a 
  prominent 
  umbo, 
  the 
  beak 
  being 
  

   closely 
  incurved 
  over 
  the 
  hinge, 
  concealing 
  both 
  foramen 
  and 
  

   deltidium. 
  The 
  umbo 
  is 
  longitudinally 
  keeled, 
  but 
  at 
  about 
  one- 
  

  

  Fig. 
  334. 
  ig. 
  385. 
  

  

  The 
  brachidium 
  of 
  Cyclospira 
  bisulcata, 
  Emmons 
  (sp.). 
  

  

  third 
  the 
  length 
  of 
  the 
  valve 
  a 
  median 
  furrow 
  begins 
  on 
  this 
  

   ridge, 
  widening 
  anteriorly, 
  and 
  thus 
  making 
  a 
  double 
  keel 
  over 
  

   the 
  forward 
  parts 
  of 
  the 
  shell. 
  The 
  lateral 
  slopes 
  are 
  broad 
  

   and 
  smooth, 
  interrupted 
  only 
  in 
  the 
  umbo-lateral 
  regions 
  by 
  

   a 
  short 
  fold 
  on 
  each 
  side, 
  originating 
  at 
  the 
  beak 
  and 
  lying 
  just 
  

   within 
  the 
  margins. 
  

  

  The 
  brachial 
  valve 
  is 
  slightly 
  convex 
  posteriorly, 
  becoming 
  

   concave 
  medially 
  over 
  the 
  pallial 
  region. 
  The 
  median 
  sinus 
  

  

  60 
  

  

  