﻿Si 
  

  

  - 
  

  

  ■ 
  

  

  PLA 
  TE 
  XXXI 
  — 
  ( 
  Continued). 
  

  

  well 
  marked, 
  with 
  an 
  exterior 
  thickening 
  of 
  loose 
  texture 
  ; 
  and 
  in 
  another 
  

   specimen, 
  the 
  inner 
  wall 
  is 
  similarly 
  well-defined, 
  with 
  an 
  exterior 
  thickening 
  

   of 
  the 
  substance 
  which 
  is 
  distinctly 
  vesicular 
  in 
  structure. 
  

  

  Spirorbis 
  inornatus 
  Hall. 
  

   Page 
  181. 
  

  

  Fig. 
  14. 
  A 
  group 
  of 
  three 
  specimens, 
  natural 
  size, 
  attached 
  to 
  the 
  surface 
  of 
  

  

  a 
  Strophomena. 
  

   Fig. 
  15. 
  A 
  specimen 
  enlarged, 
  to 
  show 
  the 
  character 
  of 
  the 
  shell 
  and 
  the 
  

  

  incipient 
  annulations. 
  

  

  

  

  ' 
  

  

  

  