﻿The 
  Fauna 
  of 
  the 
  Niagaka 
  Group. 
  

  

  167 
  

  

  Fig. 
  2. 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  4. 
  

  

  <? 
  

  

  Kg. 
  1. 
  Dorsal 
  view, 
  showing 
  extent 
  and 
  divergence 
  of 
  the 
  crural 
  processes 
  c, 
  

  

  and 
  one 
  of 
  the 
  articulations 
  of 
  the 
  ventral 
  valve 
  t. 
  

   Fig. 
  2. 
  Lateral 
  view, 
  showing 
  curvature 
  and 
  direction 
  of 
  crural 
  processes. 
  

   Fig. 
  3. 
  Enlargement 
  to 
  four 
  diameters 
  of 
  the 
  beak, 
  showing 
  the 
  characters 
  of 
  

  

  the 
  deltidium 
  and 
  foramen. 
  

   Fig. 
  4. 
  Interior 
  of 
  ventral 
  valve, 
  and 
  a 
  portion 
  of 
  the 
  dorsal 
  valve, 
  showing 
  

   the 
  attachment 
  of 
  the 
  crurae. 
  

  

  c. 
  Crurae 
  uniting 
  by 
  a 
  loop. 
  

   p. 
  Hinge-plate, 
  or 
  pedicle 
  sheath. 
  

   t. 
  Articulations. 
  

   The 
  additional 
  features 
  of 
  the 
  loop 
  represented 
  in 
  this 
  figure 
  have 
  not, 
  as 
  yet, 
  

   been 
  satisfactorily 
  determined 
  ; 
  all 
  the 
  positive 
  evidence 
  indicates 
  this 
  to 
  be 
  the 
  

   arrangement 
  and 
  development 
  of 
  the 
  parts 
  shown 
  in 
  figs. 
  1 
  and 
  2. 
  

  

  Rhynchotreta 
  cttneata 
  var. 
  Americana 
  n. 
  var. 
  

  

  Plate 
  25, 
  Figs. 
  29-38. 
  

  

  Shell 
  triangular, 
  cuneiform, 
  longer 
  than 
  wide, 
  greatest 
  

   width 
  near 
  the 
  front, 
  and 
  tapering 
  posteriorly 
  into 
  an 
  angular 
  

   beak. 
  Yalves 
  moderately 
  convex, 
  the 
  dorsal 
  sometimes 
  gib- 
  

   bous 
  ; 
  ventral 
  beak 
  elongated, 
  foramen 
  subcircular, 
  formed 
  

   by 
  the 
  extremity 
  of 
  the 
  beak 
  and 
  a 
  portion 
  of 
  the 
  area 
  below, 
  

   which 
  is 
  separated 
  from 
  the 
  hinge-line 
  by 
  a 
  deltidium 
  in 
  two 
  

   pieces 
  ; 
  sides 
  of 
  the 
  beak 
  compressed, 
  flat 
  or 
  concave. 
  Sinus 
  

   wide, 
  deep 
  or 
  shallow, 
  according 
  to 
  the 
  development 
  of 
  the 
  

   shell, 
  commencing 
  at 
  one-third 
  the 
  length 
  of 
  the 
  shell 
  from 
  

   the 
  beak, 
  and 
  becoming 
  very 
  conspicuous 
  in 
  front. 
  Dorsal 
  

   valve 
  the 
  more 
  convex, 
  the 
  mesial 
  fold 
  beginning 
  as 
  a 
  depres- 
  

   sion 
  just 
  below 
  the 
  beak, 
  and 
  becoming 
  very 
  prominent 
  on 
  

   the 
  lower 
  half 
  of 
  the 
  shell. 
  Surface 
  marked 
  by 
  nine 
  or 
  ten 
  

   strong 
  angular 
  plications 
  on 
  each 
  valve, 
  of 
  which 
  three 
  are 
  

   depressed 
  in 
  the 
  sinus, 
  and 
  four 
  are 
  elevated 
  on 
  the 
  mesial 
  

   fold 
  — 
  the 
  two 
  central 
  ones 
  being 
  much 
  the 
  more 
  prominent 
  ; 
  

   the 
  plications 
  are 
  crossed 
  by 
  numerous, 
  regular, 
  fine 
  thread- 
  

   like 
  striae. 
  The 
  entire 
  surface 
  is 
  minutely 
  papillose. 
  

  

  