﻿970 
  Forty- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  or 
  flanges, 
  beginning 
  at 
  the 
  hinge-line, 
  having 
  a 
  slightly 
  sinuous 
  

   curve 
  and 
  terminating 
  before 
  traversing 
  much 
  more 
  than 
  one- 
  

   third 
  the 
  length 
  of 
  the 
  valve 
  ; 
  occasionally 
  there 
  is 
  a 
  second 
  of 
  

   these 
  ridges 
  on 
  each 
  side. 
  

  

  The 
  muscular 
  impressions 
  of 
  the 
  valves 
  are 
  essentially 
  as 
  in 
  

   Mekistella. 
  The 
  hinge-plate 
  has 
  a 
  peculiar 
  structure; 
  it 
  arises 
  

   vertically 
  from 
  the 
  bottom 
  of 
  the 
  valve, 
  presenting 
  an 
  erect, 
  con- 
  

   cave 
  anterior 
  face, 
  which 
  is 
  traversed 
  by 
  a 
  faint 
  median 
  ridge 
  

   continuous 
  with 
  the 
  septum 
  of 
  the 
  valve. 
  The 
  posterior 
  portion 
  

   of 
  the 
  upper 
  surface 
  of 
  this 
  plate 
  bears 
  a 
  deep 
  circular 
  or 
  

   crescentic 
  concavity, 
  most 
  sharply 
  defined 
  on 
  its 
  anterior 
  edge 
  

   where 
  it 
  is 
  bounded 
  by 
  the 
  somewhat 
  recurved 
  vertical 
  wall. 
  On 
  

   the 
  lateral 
  portions 
  of 
  the 
  upper 
  face 
  of 
  the 
  anterior 
  wall 
  lie 
  the 
  

   elongate 
  crural 
  bases 
  which 
  are 
  continued 
  irto 
  short, 
  straight 
  

   crura, 
  standing 
  at 
  an 
  angle 
  of 
  about 
  forty 
  five 
  degrees 
  to 
  the 
  

   plane 
  of 
  the 
  horizontal 
  face 
  of 
  the 
  plate. 
  The 
  spiral 
  cones 
  are 
  

   as 
  in 
  Meeistella, 
  their 
  curvature 
  conforming 
  to 
  the 
  peculiarly 
  

   contracted 
  interior 
  cavity 
  of 
  the 
  shell. 
  Precise 
  nature 
  of 
  the 
  

   jugum 
  not 
  known. 
  

  

  Type, 
  Pentagonia 
  unisulcata, 
  Conrad 
  (sp.). 
  

  

  Distribution. 
  Devonian 
  (Upper 
  Helderberg 
  and 
  Hamilton 
  

   groups). 
  

  

  Camarospira, 
  Hall. 
  1893. 
  

   (Plate 
  33, 
  figs. 
  20-23.) 
  

  

  Shells 
  essentially 
  meristelloid 
  in 
  external 
  and 
  internal 
  

   characters. 
  The 
  important 
  difference 
  from 
  allied 
  genera 
  lies 
  in 
  

   the 
  fact 
  that 
  the 
  dental 
  plates 
  of 
  the 
  pedicle-valve, 
  instead 
  of 
  

   resting 
  upon 
  the 
  bottom 
  of 
  the 
  valve, 
  are 
  more 
  strongly 
  conver- 
  

   gent 
  than 
  in 
  Merista, 
  Mekistella, 
  etc., 
  uniting 
  before 
  they 
  reach 
  

   the 
  internal 
  surface 
  of 
  the 
  valve, 
  thus 
  restricting 
  the 
  impression 
  

   of 
  the 
  pedicle- 
  muscle 
  to 
  a 
  distinct 
  chamber 
  or 
  spondylium, 
  which 
  

   is 
  supported 
  by 
  a 
  low 
  median 
  septum. 
  In 
  the 
  typical 
  species 
  

   this 
  chamber 
  has 
  the 
  same 
  extent 
  as 
  the 
  deep 
  pedicle-cavity 
  in 
  

   Mekistella, 
  that 
  is, 
  about 
  one-fourth 
  the 
  length 
  of 
  the 
  valve, 
  

   while 
  the 
  septum 
  extends 
  for 
  a 
  short 
  distance 
  beyond 
  its 
  anterior 
  

   margin, 
  dividing 
  the 
  scars 
  of 
  the 
  adductor 
  and 
  diductor 
  

   impressions. 
  In 
  this 
  respect 
  the 
  internal 
  structure 
  of 
  this 
  valve 
  

   is 
  similar 
  to 
  that 
  of 
  the 
  corresponding 
  valve 
  of 
  Pentamerus. 
  

  

  