﻿The 
  Fauna 
  of 
  the 
  Niagara 
  Group. 
  153 
  

  

  not 
  distinctly 
  marked. 
  The 
  inner 
  cardinal 
  margins 
  of 
  the 
  

   valves 
  are 
  marked 
  by 
  about 
  fifteen 
  slightly 
  diverging 
  crenu- 
  

   lations 
  on 
  each 
  side 
  of 
  the 
  foramen, 
  which 
  occupy 
  little 
  more 
  

   than 
  one-third 
  of 
  the 
  distance 
  from 
  the 
  center 
  to 
  the 
  extremi- 
  

   ties 
  of 
  the 
  shell. 
  

  

  STROPHOKELLA 
  n. 
  gen. 
  

  

  On 
  several 
  occasions, 
  and 
  notably 
  in 
  the 
  Sixteenth 
  Report 
  

   on 
  the 
  State 
  Cabinet 
  of 
  Natural 
  History, 
  and 
  in 
  Yol. 
  IV 
  of 
  the 
  

   Palaeontology 
  of 
  New 
  York, 
  I 
  have 
  indicated 
  the 
  existence 
  

   of 
  a 
  small 
  group 
  of 
  strophomenoid 
  shells 
  which 
  are 
  not 
  

   strictly 
  included 
  in 
  the 
  genera 
  Strophomena, 
  Strophodonta 
  

   or 
  Streptorhynchus, 
  though 
  intimately 
  related 
  to 
  these 
  

   genera. 
  The 
  features 
  are 
  essentially 
  those 
  of 
  Streptorhyn- 
  

   chus 
  in 
  the 
  resupinate 
  position 
  of 
  the 
  valves, 
  and 
  to 
  a 
  great 
  

   extent 
  in 
  the 
  muscular 
  impressions 
  ; 
  but 
  the 
  inner 
  margins 
  of 
  

   the 
  cardinal 
  areas 
  are 
  crenulate, 
  and 
  the 
  area 
  usually 
  solid 
  with 
  

   sometimes 
  a 
  triangular 
  deltidium. 
  These 
  forms 
  are 
  separated 
  

   from 
  Strophodonta 
  chiefly 
  by 
  their 
  resupinate 
  character, 
  the 
  

   strong 
  and 
  more 
  restricted 
  muscular 
  impression 
  of 
  the 
  ventral 
  

   valve, 
  which 
  occupies 
  a 
  shorter 
  and 
  wider 
  area, 
  and 
  by 
  the 
  

   muscular 
  area 
  and 
  strong 
  median 
  septum 
  in 
  the 
  dorsal 
  valve. 
  

   The 
  cardinal 
  process 
  is 
  similar 
  to 
  that 
  of 
  Strophodonta 
  and 
  

   also 
  to 
  some 
  of 
  the 
  Devonian 
  forms 
  of 
  Streptorhynchus. 
  

   The 
  characteristic 
  features 
  of 
  these 
  forms 
  are 
  as 
  follows 
  : 
  

  

  Shell 
  semicircular 
  or 
  semielliptical, 
  concavo-convex, 
  resu- 
  

   pinate, 
  the 
  ventral 
  valve 
  concave 
  and 
  the 
  dorsal 
  valve 
  con- 
  

   vex. 
  Ventral 
  area 
  striated, 
  solid, 
  with 
  or 
  without 
  a 
  central 
  

   deltidial 
  scar 
  or 
  rarely 
  a 
  partial 
  foramen, 
  and 
  similar 
  fea- 
  

   tures 
  on 
  the 
  narrow 
  area 
  of 
  the 
  dorsal 
  valve 
  ; 
  inner 
  margins 
  of 
  

   the 
  cardinal 
  areas 
  of 
  each 
  valve 
  crenulate, 
  and 
  from 
  beneath 
  

   the 
  center 
  of 
  the 
  ventral 
  area 
  there 
  is 
  often 
  a 
  strong 
  process 
  

   (frequently 
  bilobed) 
  which 
  extends 
  beyond 
  the 
  cardinal 
  line. 
  

   Muscular 
  area 
  of 
  the 
  ventral 
  valve 
  strongly 
  marked, 
  and 
  lim- 
  

   ited 
  by 
  a 
  prominent 
  border. 
  Dorsal 
  valve 
  with 
  a 
  narrow 
  

   hinge 
  area 
  transversely 
  or 
  longitudinally 
  striate 
  or 
  both, 
  and 
  

   marked 
  in 
  the 
  center 
  by 
  a 
  deltidial 
  scar. 
  Cardinal 
  process 
  

   double, 
  each 
  division 
  notched 
  or 
  bidentate 
  at 
  the 
  extremity 
  ; 
  

   muscular 
  area 
  quadrangular, 
  occupying 
  a 
  more 
  or 
  less 
  ele- 
  

   vated 
  callosity, 
  and 
  a 
  central 
  carina 
  rising 
  from 
  the 
  lower 
  

   part 
  of 
  this 
  area 
  is 
  sometimes 
  produced 
  into 
  a 
  spiniform 
  pro- 
  

   cess 
  in 
  the 
  center 
  of 
  the 
  cavity. 
  

  

  