﻿72 
  JIE. 
  E. 
  E. 
  COWPEB 
  EEED 
  OS 
  THE 
  [Feb. 
  1 
  897, 
  

  

  furnished 
  with 
  a 
  single 
  median 
  rib. 
  The 
  brachial 
  valve 
  has 
  a 
  low 
  

   median 
  fold, 
  marked 
  by 
  two 
  ribs 
  and 
  a 
  somewhat 
  wide 
  interspace 
  

   on 
  each 
  side 
  between 
  the 
  fold 
  and 
  the 
  lateral 
  group 
  of 
  ribs.' 
  The 
  

   cardinal 
  angles 
  of 
  the 
  shell 
  are 
  more 
  rounded 
  than 
  in 
  the 
  type-form. 
  

   I 
  was 
  at 
  first 
  inclined 
  to 
  remove 
  this 
  form 
  into 
  a 
  new 
  species, 
  but 
  

   perhaps 
  it 
  is 
  safer 
  at 
  present 
  merely 
  to 
  consider 
  it 
  a 
  variety 
  of 
  

   A. 
  expansa. 
  It 
  apparently 
  occurs 
  in 
  the 
  Leptcena-L'imestoTie. 
  

  

  Ateypa 
  maeginalis 
  (Dalman). 
  

  

  There 
  is 
  no 
  reason 
  to 
  doubt 
  that 
  this 
  species 
  or 
  a 
  variety 
  of 
  it 
  

   occurs 
  in 
  the 
  Keisley 
  Limestone, 
  as 
  its 
  characters 
  are 
  sufficiently 
  

   distinct 
  from 
  those 
  of 
  A. 
  expansa 
  in 
  its 
  typical 
  form. 
  

  

  Ateypina 
  similis, 
  sp. 
  n. 
  (PL 
  VI. 
  figs. 
  2, 
  2 
  a, 
  2 
  6, 
  3, 
  & 
  3 
  a.) 
  

  

  Shell 
  small, 
  subcircular, 
  retziform, 
  flattened. 
  Pedicle 
  -valve 
  more 
  

   convex 
  than 
  the 
  brachial 
  valve, 
  and 
  furnished 
  with 
  a 
  prominent 
  beak 
  

   pierced 
  by 
  a 
  distinct 
  circular 
  foramen 
  with 
  a 
  deltidium 
  (of 
  one 
  piece 
  ?) 
  

   in 
  front. 
  The 
  surface 
  of 
  this 
  valve 
  is 
  traversed 
  by 
  seven 
  radiating 
  

   rounded 
  ribs 
  extending 
  from 
  the 
  beak 
  to 
  the 
  margin 
  ; 
  of 
  these 
  the 
  

   median 
  one 
  bifurcates 
  close 
  to 
  the 
  beak 
  into 
  two 
  strong 
  rounded 
  

   ribs, 
  each 
  of 
  which 
  again 
  bifurcates 
  at 
  about 
  half 
  its 
  length. 
  The 
  

   other 
  ribs 
  are 
  simple, 
  and 
  decrease 
  in 
  strength 
  near 
  the 
  hinge-line. 
  

  

  Brachial 
  valve 
  flattened, 
  more 
  convex 
  posteriorly, 
  with 
  small 
  

   ircmspicuous 
  beak 
  and 
  shallow 
  mesial 
  sinus, 
  widening 
  anteriorly. 
  

   In 
  this 
  sinus 
  lies 
  a 
  single 
  rounded 
  rib 
  which 
  arises 
  at 
  a 
  point 
  about 
  

   half 
  the 
  length 
  of 
  the 
  shell 
  and 
  bifurcates 
  into 
  two 
  weaker 
  ribs 
  

   which 
  reach 
  the 
  margin. 
  On 
  each 
  side 
  of 
  the 
  sinus 
  is 
  a 
  strong 
  

   rounded 
  rib, 
  arising 
  from 
  the 
  beak 
  and 
  bifurcating, 
  at 
  about 
  half 
  its 
  

   length, 
  into 
  two 
  weaker 
  contiguous 
  ribs 
  reaching 
  the 
  margin 
  like 
  

   the 
  central 
  one 
  in 
  the 
  sinus. 
  (In 
  the 
  young 
  form, 
  PI. 
  VI. 
  fig. 
  2 
  a, 
  

   these 
  ribs 
  do 
  not 
  bifurcate.) 
  On 
  each 
  lateral 
  portion 
  of 
  the 
  valve 
  

   outside 
  these 
  ribs 
  bordering 
  the 
  sinus 
  are 
  three 
  simple 
  ribs, 
  those 
  

   nearest 
  the 
  hinge-line 
  being 
  the 
  weakest. 
  

  

  raillim. 
  

  

  Length 
  of 
  shell 
  5 
  5 
  

  

  Breadth 
  5'0 
  

  

  This 
  species 
  much 
  resembles 
  Atrypa 
  Barrandei 
  (Davidson) 
  l 
  in 
  

   shape 
  and 
  general 
  characters, 
  as 
  well 
  as 
  in 
  the 
  distribution 
  of 
  the 
  

   ribs 
  on 
  each 
  valve. 
  But 
  it 
  differs 
  in 
  the 
  bifurcation 
  of 
  certain 
  of 
  

   these 
  ribs 
  and 
  in 
  the 
  absence 
  of 
  the 
  concentric-growth 
  ridges. 
  

  

  Hall 
  & 
  Clarke 
  2 
  have 
  recently 
  designated 
  the 
  group 
  of 
  shells 
  to 
  

   which 
  Atrypa 
  Barrandei 
  belongs 
  by 
  the 
  generic 
  term 
  of 
  Atrypina. 
  

   Our 
  Keisley 
  species 
  is 
  especially 
  closely 
  allied 
  to 
  A. 
  disparilis 
  

   (Hall) 
  3 
  of 
  the 
  Niagara 
  Group, 
  and 
  also 
  much 
  resembles 
  the 
  type 
  of 
  

   the 
  genus 
  A. 
  imbricata 
  (Hail) 
  4 
  from 
  the 
  Lower 
  Helderberg 
  Group. 
  

  

  1 
  Davidson, 
  <Mon. 
  Brit. 
  Foss. 
  Brach.,' 
  vol. 
  iii. 
  (1864-71) 
  p. 
  128, 
  pi. 
  xiii. 
  

   figs. 
  10-13; 
  vol. 
  v. 
  Suppl. 
  Sil. 
  Brach. 
  (1882) 
  p. 
  114, 
  pi. 
  vii. 
  figs. 
  7-7 
  b. 
  

  

  2 
  < 
  Pal. 
  N. 
  Y.,' 
  vol. 
  viii. 
  Brach. 
  pt. 
  ii. 
  (1894) 
  p. 
  161. 
  

  

  3 
  Ibid. 
  vol. 
  ii. 
  (1852) 
  p. 
  .277, 
  pi. 
  lvii. 
  fig. 
  6. 
  

  

  4 
  Ibid. 
  vol. 
  iii. 
  (1859) 
  p. 
  246, 
  pi. 
  xxxviii. 
  figs. 
  8-13. 
  , 
  

  

  