﻿The 
  Fauna 
  of 
  the 
  Niagara 
  Group. 
  161 
  

  

  tinotly 
  rounded, 
  with 
  a 
  visible 
  triangular 
  space 
  below, 
  which 
  is 
  

   occupied 
  by 
  two 
  small 
  deltidial 
  plates. 
  Dorsal 
  valve 
  regularly 
  

   arcuate 
  except 
  near 
  the 
  front 
  ; 
  beak 
  closely 
  incurved 
  beneath 
  

   the 
  deltidial 
  plates 
  of 
  the 
  opposite 
  valve. 
  In 
  some 
  old 
  speci- 
  

   mens 
  there 
  is 
  a 
  broad 
  undefined 
  mesial 
  elevation 
  on 
  the 
  lower 
  

   part 
  of 
  the 
  valve 
  including 
  about 
  five 
  or 
  six 
  plications 
  besides 
  

   the 
  central 
  one, 
  which 
  is 
  divided 
  into 
  two 
  or 
  three 
  smaller 
  

   ones 
  (a 
  generic 
  feature). 
  Sometimes 
  there 
  is 
  a 
  broad, 
  unde- 
  

   fined 
  depression 
  and 
  frequently 
  only 
  a 
  narrow 
  depression 
  

   caused 
  by 
  the 
  subdivided 
  central 
  plication. 
  The 
  ventral 
  valve 
  

   has 
  uniformly 
  a 
  longitudinal 
  sinus, 
  which 
  includes 
  two 
  or 
  

   three 
  small 
  plications 
  arising 
  from 
  the 
  subdivision 
  of 
  the 
  cen- 
  

   tral 
  one, 
  and 
  sometimes 
  including 
  one 
  or 
  two 
  on 
  each 
  side. 
  

  

  This 
  shell 
  is 
  marked 
  by 
  radiating 
  rounded 
  or 
  subangular 
  

   costae 
  or 
  plications, 
  from 
  eight 
  to 
  fourteen 
  on 
  each 
  side 
  of 
  the 
  

   central 
  one 
  ; 
  those 
  on 
  the 
  cardinal 
  slopes 
  sometimes 
  bifurcat- 
  

   ing 
  or 
  with 
  interstitial 
  additions, 
  while 
  in 
  a 
  few 
  individuals 
  

   bifurcating 
  costse 
  occur 
  on 
  other 
  parts 
  of 
  the 
  valve 
  ; 
  the 
  inter- 
  

   spaces 
  are 
  rounded 
  grooves 
  of 
  about 
  the 
  same 
  size 
  as 
  the 
  pli- 
  

   cations. 
  The 
  surface 
  is 
  marked 
  by 
  fine 
  concentric 
  striae 
  and 
  

   stronger, 
  imbricating 
  lamellose 
  lines 
  of 
  growth. 
  

  

  This 
  species 
  differs 
  from 
  R. 
  (Atrypa) 
  apriiiis 
  of 
  the 
  Niagara 
  

   group 
  of 
  New 
  York, 
  in 
  being 
  more 
  ovate 
  in 
  form 
  and 
  less 
  

   gibbous 
  toward' 
  the 
  front, 
  while 
  the 
  ventral 
  beak 
  is 
  more 
  

   elevated. 
  It 
  approaches 
  very 
  nearly 
  in 
  character 
  to 
  the 
  M. 
  

   formosa 
  of 
  the 
  Lower 
  Helderberg 
  group, 
  and 
  the 
  shell 
  is 
  sub- 
  

   ject 
  to 
  similar 
  variations 
  of 
  form, 
  proportions 
  and 
  incurvation 
  

   of 
  the 
  beak, 
  which 
  in 
  some 
  specimens 
  of 
  both 
  species 
  is 
  closed 
  

   upon 
  the 
  opposite 
  beak. 
  The 
  central 
  plication 
  in 
  both 
  species 
  

   is 
  divided 
  into 
  two 
  or 
  three, 
  and 
  the 
  dorsal 
  valve 
  likewise 
  

   presents 
  sometimes 
  a 
  mesial 
  elevation 
  and 
  sometimes 
  a 
  depres- 
  

   sion. 
  

  

  The 
  largest 
  individual 
  measured 
  has 
  a 
  length 
  of 
  more 
  than 
  

   25 
  mm., 
  with 
  a 
  width 
  nearly 
  as 
  great. 
  The 
  specimens 
  vary 
  

   from 
  2 
  mm. 
  to 
  about 
  25 
  mm. 
  in 
  length. 
  

  

  This 
  species 
  is 
  comparatively 
  numerous. 
  

  

  17 
  

  

  