﻿Report 
  of 
  the 
  State 
  Geologist. 
  1043 
  

  

  Rensselaeria, 
  Hall. 
  1859. 
  

   (Plate 
  46, 
  figs. 
  1-15.) 
  

  

  Shells 
  ovate 
  or 
  elongate-ovate 
  in 
  outline, 
  subovoid 
  in 
  contour, 
  

   broadest 
  posteriorly 
  ; 
  valves 
  unequally 
  convex, 
  with 
  a 
  more 
  or 
  

   less 
  distinct 
  median 
  elevation 
  on 
  both. 
  Cardinal 
  slopes 
  broadly 
  

   flattened 
  in 
  the 
  typical 
  species. 
  Lateral 
  margins 
  compressed, 
  

   often 
  strongly 
  inflected. 
  

  

  Pedicle-valve 
  with 
  full, 
  scarcely 
  salient 
  umbo 
  ; 
  beak 
  acute, 
  

   incurved, 
  with 
  terminal 
  foramen. 
  Beneath 
  and 
  on 
  each 
  side 
  of 
  

   the 
  beak 
  is 
  a 
  concave, 
  sharply 
  defined 
  space, 
  but 
  no 
  proper 
  cardi- 
  

   nal 
  area. 
  Distinct 
  deltidial 
  plates 
  divided 
  by 
  a 
  median 
  suture 
  

   may 
  be 
  present, 
  but 
  they 
  are 
  usually 
  concealed 
  by 
  incurvature 
  or 
  

   atrophied 
  by 
  the 
  encroachment 
  of 
  the 
  umbo 
  of 
  the 
  opposite 
  

   valve. 
  On 
  the 
  interior 
  the 
  teeth 
  are 
  prominent 
  but 
  not 
  thickened, 
  

   widely 
  divergent 
  and 
  close 
  within 
  the 
  margins 
  of 
  the 
  valve. 
  

   They 
  are 
  supported 
  by 
  stout 
  dental 
  plates 
  which 
  rest 
  for 
  most 
  of 
  

   their 
  length 
  upon 
  the 
  bottom 
  of 
  the 
  valve, 
  but 
  are 
  free 
  along 
  

   their 
  posterior 
  margins. 
  These 
  plates 
  are 
  closely 
  appressed 
  

   against 
  the 
  lateral 
  walls 
  of 
  the 
  shell 
  and 
  become 
  coalescent 
  there- 
  

   with 
  in 
  later 
  growth-stages. 
  The 
  muscular 
  impression 
  is 
  restricted 
  

   to 
  the 
  posterior 
  portion 
  of 
  the 
  valve, 
  covering 
  a 
  very 
  narrow 
  lon- 
  

   gitudinal 
  area. 
  Between 
  the 
  dental 
  plates 
  at 
  the 
  bottom 
  of 
  the 
  

   pedicle-cavity 
  lies 
  a 
  narrow 
  median 
  scar, 
  and 
  in 
  front 
  of 
  this, 
  a 
  

   very 
  deep, 
  elongate 
  impression, 
  which 
  is 
  sometimes 
  clearly 
  divided 
  

   into 
  central 
  adductor 
  scars 
  embraced 
  by 
  the 
  diductor 
  impressions. 
  

   Over 
  the 
  post-lateral 
  slopes 
  are 
  numerous 
  fine, 
  irregularly 
  rami- 
  

   fying 
  sinuses. 
  

  

  The 
  brachial 
  valve 
  is 
  considerably 
  the 
  less 
  convex 
  and 
  is 
  often 
  

   flattened. 
  The 
  beak 
  is 
  minute 
  and 
  usually 
  obscured 
  by 
  the 
  over- 
  

   lapping 
  pedicle 
  -valve. 
  The 
  hinge 
  plate 
  is 
  large, 
  flat, 
  triangular, 
  

   sometimes 
  thin, 
  often 
  thickened 
  on 
  its 
  posterior 
  portion 
  and 
  rest- 
  

   ing 
  on 
  the 
  bottom 
  of 
  the 
  valve. 
  It 
  is 
  separated 
  from 
  the 
  lateral 
  

   shell-walls 
  by 
  narrow 
  dental 
  grooves, 
  widening 
  at 
  their 
  extremi- 
  

   ties. 
  Normally 
  this 
  plate 
  is 
  perforated 
  by 
  a 
  visceral 
  foramen 
  

   entering 
  at 
  the 
  under 
  side 
  and 
  opening 
  at 
  or 
  beneath 
  the 
  apex 
  of 
  

   the 
  beak, 
  but 
  usually 
  this 
  perforation 
  is 
  closed 
  by 
  adventitious 
  

   deposits. 
  The 
  crura 
  are 
  continuations 
  of 
  the 
  upper 
  portions 
  of 
  

   the 
  supporting 
  septa 
  ; 
  they 
  are 
  broadened 
  just 
  beyond 
  their 
  base 
  

   of 
  attachment, 
  and 
  from 
  their 
  upper 
  angles 
  are 
  given 
  off 
  the 
  

  

  101 
  " 
  

  

  