﻿G. 
  0. 
  Dunbar 
  — 
  Rensselterina, 
  a 
  New 
  Genus. 
  469 
  

  

  Such 
  a 
  genus 
  in 
  the 
  early 
  Mississippian, 
  however, 
  can 
  

   scarcely 
  have 
  evolved 
  from 
  one 
  which 
  in 
  the 
  early 
  Devonian 
  

   has 
  already 
  reached 
  as 
  great 
  a 
  specialization 
  in 
  another 
  direc- 
  

   tion 
  as 
  is 
  the 
  case 
  in 
  Rensselcerina. 
  In 
  the 
  latter 
  genus 
  the 
  

   vertical 
  median 
  lamella 
  is 
  simple, 
  not 
  double, 
  it 
  has 
  no 
  posterior 
  

   extension, 
  and 
  it 
  is 
  not 
  formed 
  by 
  a 
  deflection 
  of 
  the 
  descend- 
  

   ing 
  lamellae 
  of 
  the 
  loop. 
  On 
  the 
  contrary, 
  the 
  halves 
  of 
  the 
  

   loop 
  meet 
  medially 
  without 
  any 
  deflection 
  or 
  thickening, 
  and 
  

   the 
  dorsal 
  side 
  of 
  the 
  median 
  plate 
  is 
  smooth 
  and 
  evenly 
  

   rounded, 
  while 
  from 
  its 
  ventral 
  side 
  there 
  arises 
  the 
  very 
  thin, 
  

   simple, 
  vertical 
  lamella. 
  

  

  Summary. 
  — 
  The 
  essential 
  characters 
  of 
  the 
  new 
  genus 
  

   Rensselcerina 
  will 
  be, 
  then, 
  its 
  rensselseroid 
  shape, 
  its 
  medially 
  

   plicate 
  surface, 
  and, 
  on 
  its 
  interior, 
  the 
  centronelloid 
  brachidium 
  

   with 
  its 
  high, 
  vertical, 
  anteriorly 
  directed, 
  median 
  lamella. 
  

  

  Rensselcerina 
  medioplicata, 
  new 
  species. 
  

   PI. 
  II, 
  Figs. 
  1-9, 
  12-18. 
  

  

  Shell 
  with 
  the 
  characters 
  of 
  the 
  genus 
  ; 
  length 
  of 
  a 
  mature 
  

   individual 
  24 
  mm 
  , 
  width 
  14 
  mm 
  , 
  thickness 
  ll 
  mm 
  . 
  Six 
  to 
  eight 
  

   plications 
  on 
  the 
  middle 
  of 
  the 
  ventral 
  valve 
  and 
  one 
  more 
  on 
  

   the 
  dorsal. 
  About 
  three 
  of 
  these 
  plications 
  in 
  a 
  width 
  of 
  5 
  mm 
  

   on 
  the 
  anterior 
  margin. 
  Hinge-plate 
  extremely 
  variable, 
  being 
  

   in 
  some 
  specimens 
  excavate, 
  in 
  others 
  raised 
  into 
  a 
  median 
  

   process. 
  

  

  Horizon 
  and 
  locality. 
  — 
  Linden 
  formation 
  ; 
  western 
  Ten- 
  

   nessee. 
  The 
  cotypes 
  are 
  in 
  the 
  Peabody 
  Museum, 
  Yale 
  

   University. 
  

  

  Rensselcerina 
  medioplicata 
  var. 
  latior, 
  new 
  variety. 
  

   PI. 
  II, 
  Figs. 
  10, 
  11. 
  

  

  In 
  Professor 
  Schuchert's 
  collection 
  there 
  is 
  a 
  shell 
  like 
  

   R. 
  medioplicata 
  but 
  proportionally 
  much 
  broader 
  and 
  shorter, 
  

   and 
  with 
  the 
  plications 
  liner 
  and 
  somewhat 
  more 
  restricted 
  to 
  

   the 
  anterior 
  part 
  of 
  the 
  shell. 
  The 
  length 
  of 
  a 
  fully 
  grown 
  

   specimen 
  is 
  20 
  mm 
  , 
  breadth 
  16 
  Ium 
  , 
  thickness 
  ll 
  mm 
  . 
  There 
  are 
  

   four 
  plications 
  in 
  about 
  the 
  width 
  of 
  three 
  in 
  R. 
  medioplicata. 
  

  

  Horizon 
  and 
  locality. 
  — 
  Linden 
  formation 
  ; 
  Henry 
  County, 
  

   Tennessee. 
  Holotype 
  in 
  Professor 
  Schuchert's 
  collection. 
  

  

  