﻿The 
  Fatoa 
  of 
  the 
  Niagara 
  Group. 
  163 
  

  

  Rhynchonella 
  acinus. 
  

  

  Plate 
  26, 
  Figs. 
  7-11. 
  

  

  Rhynchonella 
  acinus 
  Hall. 
  Trans. 
  Alb. 
  Inst., 
  vol. 
  iv, 
  p. 
  215. 
  Abstract, 
  p. 
  21; 
  

   May, 
  1863. 
  

  

  Shell 
  small, 
  longitudinally 
  ovate, 
  subattenuate 
  toward 
  the 
  

   beak, 
  a 
  ad 
  truncate 
  in 
  front, 
  valves 
  subequally 
  convex. 
  Ven- 
  

   tral 
  valve 
  subarcuate, 
  flattened 
  in 
  the 
  middle, 
  below 
  which 
  it 
  

   is 
  sinuate 
  ; 
  beak 
  incurved. 
  Dorsal 
  valve 
  somewhat 
  flattened 
  

   in 
  the 
  middle, 
  and 
  sometimes 
  a 
  little 
  depressed 
  in 
  the 
  upper 
  

   part 
  of 
  the 
  median 
  line, 
  two 
  of 
  the 
  plications 
  becoming 
  eleva- 
  

   ted 
  toward 
  the 
  front, 
  corresponding 
  to 
  an 
  abruptly 
  depressed 
  

   sinus 
  in 
  the 
  ventral 
  valve, 
  in 
  the 
  bottom 
  of 
  which 
  is 
  a 
  single 
  

   plication 
  ; 
  three 
  and 
  rarely 
  four 
  plications 
  on 
  each 
  side 
  of 
  the 
  

   mesial 
  fold 
  of 
  the 
  dorsal 
  valve, 
  and 
  four 
  on 
  each 
  side 
  of 
  the 
  

   sinus 
  of 
  the 
  ventral 
  valve. 
  Concentric 
  lines 
  of 
  growth 
  usually 
  

   but 
  faintly 
  marked. 
  

  

  Length 
  from 
  5 
  to 
  10 
  mm.; 
  length 
  and 
  breadth 
  usually 
  about 
  

   as 
  four 
  to 
  three, 
  and 
  the 
  depth 
  about 
  equal 
  to 
  the 
  width, 
  

   giving 
  a 
  subquadrate 
  transverse 
  section. 
  

  

  This 
  species 
  diners 
  from 
  the 
  H. 
  bidentata 
  of 
  Hisinger, 
  in 
  

   being 
  larger, 
  more 
  robust 
  and 
  ventricose, 
  and 
  proportionally 
  

   more 
  elongate; 
  the 
  plications 
  are 
  more 
  rounded, 
  and 
  the 
  

   whole 
  aspect 
  less 
  angular. 
  It 
  approaches 
  in 
  form 
  the 
  H. 
  bial- 
  

   veata 
  of 
  the 
  Lower 
  Helderberg 
  group, 
  but 
  it 
  is 
  more 
  robust, 
  

   and 
  the 
  plications 
  are 
  more 
  rounded. 
  

  

  Rhynchonella 
  Indianensis. 
  

  

  Plate 
  26, 
  Figs. 
  12-22. 
  

  

  Rhynchonella 
  Indianensis 
  Hall. 
  Trans. 
  Alb. 
  Inst., 
  vol. 
  iv, 
  p. 
  215. 
  Abstract, 
  

   p. 
  21; 
  May, 
  1863. 
  

  

  Shell 
  broadly 
  ovate 
  or 
  subtriangular, 
  length 
  and 
  width 
  

   nearly 
  equal, 
  the 
  width 
  sometimes 
  exceeding 
  the 
  length 
  ; 
  car- 
  

   dinal 
  slopes 
  in 
  the 
  more 
  gibbous 
  specimens, 
  flattened. 
  Ventral 
  

   valve 
  with 
  the 
  beak 
  pointed 
  and 
  incurved, 
  depressed-convex 
  

   in 
  the 
  middle 
  and 
  gradually 
  becoming 
  depressed 
  and 
  sinuate 
  

   in 
  front, 
  two 
  or 
  three 
  of 
  the 
  plications 
  included 
  in 
  the 
  sinus. 
  

   Dorsal 
  valve 
  a 
  little 
  the 
  more 
  gibbous, 
  somewhat 
  flattened 
  

  

  