﻿470 
  C. 
  O. 
  Dunbar 
  — 
  Rensselcerina* 
  a 
  New 
  Genus. 
  

  

  EXPLANATION 
  OF 
  PLATE 
  II. 
  

  

  Rensselcerina 
  medioplicata, 
  n. 
  sp. 
  

  

  Figs. 
  1, 
  2, 
  3. 
  Dorsal, 
  side, 
  and 
  ventral 
  views 
  of 
  the 
  loop, 
  restored. 
  In 
  

   figs. 
  2 
  and 
  3 
  the 
  visceral 
  foramen 
  which 
  traverses 
  the 
  hinge-plate 
  is 
  in- 
  

   dicated. 
  

  

  Fig. 
  4, 
  Dorsal 
  view 
  of 
  the 
  posterior 
  portion 
  of 
  a 
  specimen. 
  

  

  Fig. 
  5. 
  Cardinal 
  view 
  of 
  a 
  very 
  obese 
  specimen 
  which, 
  is 
  almost 
  

   cylindrical. 
  The 
  beak 
  is 
  abraded 
  off. 
  

  

  Fig. 
  6. 
  Cardinal 
  view 
  of 
  a 
  normal 
  specimen. 
  

  

  Figs. 
  7, 
  8. 
  Lateral 
  and 
  ventral 
  views 
  of 
  a 
  specimen 
  which 
  shows 
  the 
  

   normal 
  shape 
  of 
  the 
  species. 
  Fig. 
  7 
  is 
  slightly 
  enlarged. 
  The 
  shell 
  is 
  ex- 
  

   foliated, 
  so 
  the 
  plications 
  do 
  not 
  appear 
  clearly. 
  

  

  Fig. 
  9. 
  Ventral 
  view 
  of 
  another 
  specimen, 
  to 
  show 
  that 
  the 
  posterior 
  

   portion 
  and 
  lateral 
  slopes 
  are 
  smooth 
  and 
  only 
  the 
  median 
  portion 
  is 
  

   plicated. 
  

  

  Fig. 
  12. 
  Interior 
  of 
  a 
  ventral 
  valve, 
  to 
  show 
  the 
  deep 
  pedicle 
  cavity, 
  the 
  

   hinge-teeth 
  supported 
  by 
  small 
  dental 
  lamellee, 
  and 
  the 
  narrow 
  adductor 
  

   muscle 
  scars 
  embraced 
  in 
  the 
  posterior 
  portion 
  of 
  the 
  diductor 
  muscle 
  

   scars, 
  x 
  2. 
  

  

  Fig. 
  13. 
  Interior 
  of 
  the 
  posterior 
  portion 
  of 
  a 
  dorsal 
  valve 
  which 
  shows 
  

   most 
  of 
  the 
  slender 
  adductor 
  muscle 
  scars. 
  x 
  2. 
  

  

  Figs. 
  14, 
  15, 
  16. 
  17, 
  18. 
  A 
  series 
  of 
  individuals 
  showing 
  the 
  extreme 
  

   variability 
  of 
  the 
  diductor 
  muscular 
  area 
  of 
  the 
  hinge-plate. 
  In 
  figs. 
  14 
  

   and 
  15, 
  this 
  area 
  is 
  excavate, 
  in 
  figs. 
  16 
  and 
  17 
  slightly 
  raised, 
  and 
  in 
  fig. 
  

   18 
  a 
  conspicuous 
  process 
  has 
  arisen 
  between 
  the 
  diductor 
  muscles, 
  which 
  are 
  

   inserted 
  at 
  its 
  sides. 
  Note 
  the 
  perforation 
  of 
  the 
  hinge-plate 
  by 
  the 
  visceral 
  

   foramen 
  just 
  below 
  the 
  beaks. 
  x 
  2. 
  

  

  From 
  the 
  series 
  of 
  cotypes 
  in 
  the 
  Peabody 
  Museum, 
  Yale 
  University. 
  All 
  

   figures 
  natural 
  size 
  unless 
  otherwise 
  stated. 
  

  

  Rensselcerina 
  medioplicata 
  var. 
  latior, 
  n. 
  var. 
  

  

  Figs. 
  10, 
  11. 
  Dorsal 
  and 
  lateral 
  view, 
  to 
  show 
  the 
  broad 
  and 
  short 
  out- 
  

   line 
  and 
  nearly 
  obsolete 
  plications. 
  Fig. 
  10 
  natural 
  size, 
  fig, 
  11 
  slightly 
  

   enlarged. 
  

  

  From 
  the 
  cotypes 
  in 
  Professor 
  Schuchert's 
  collection, 
  Peabody 
  Museum, 
  

   Yale 
  University. 
  

  

  