﻿i84 
  

  

  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  the 
  middle, 
  with 
  a 
  narrow 
  hinge 
  area; 
  cardinal 
  process 
  quadrilobate 
  

   as 
  seen 
  from 
  above. 
  Surface 
  covered 
  by 
  slender 
  radiating 
  striae, 
  

   which 
  are 
  crenulated 
  by 
  concentric 
  lines. 
  

  

  Orthothetes 
  subplanus 
  (Conrad) 
  (Fig. 
  91). 
  Leptaena 
  sub- 
  

   pi 
  a 
  n 
  a 
  Hall 
  (1852. 
  Pal 
  N. 
  Y. 
  2:259, 
  pi. 
  53, 
  fig. 
  8-10) 
  

  

  Distill 
  gui 
  ship. 
  g 
  characters. 
  Pedicle 
  valve 
  at 
  first 
  convex, 
  later 
  be- 
  

   coming 
  concave; 
  valves 
  nearly 
  equal 
  in 
  length 
  and 
  width; 
  extended 
  

   hinge 
  line, 
  sometimes 
  projecting 
  into 
  points; 
  sharp 
  angular 
  or 
  sub- 
  

   angular 
  to 
  rounded 
  striae, 
  sometimes 
  bifurcating 
  before 
  reaching 
  

  

  Fig. 
  91 
  Orthothetes 
  subplanus 
  

  

  margin, 
  separated 
  by 
  wider 
  interspaces 
  ; 
  the 
  usual 
  method 
  of 
  increase 
  

   is 
  by 
  intercalation 
  of 
  fine 
  striae, 
  which 
  soon 
  grow 
  to 
  strength 
  of 
  

   the 
  chief 
  ones; 
  fine 
  concentric 
  and 
  occasionally 
  coarser 
  lines 
  of 
  

   growth. 
  

  

  Found 
  at 
  Niagara 
  iA 
  the 
  upper 
  Clinton 
  beds 
  and 
  the 
  Clinton 
  

   lenses 
  ; 
  also 
  abundantly 
  in 
  certain 
  thin 
  calcareous 
  layers 
  of 
  the 
  lower 
  

  

  Rochester 
  shale, 
  less 
  common 
  in 
  

   the 
  middle 
  and 
  upper 
  shale. 
  Also 
  

   at 
  Lockport 
  and 
  elsewhere 
  (Hall). 
  

   Orthothetes 
  hydraulicus 
  (Whit- 
  

   field) 
  (Fig. 
  92) 
  (Grabau. 
  Geoi. 
  

   soc. 
  Am. 
  BuL 
  11 
  1365, 
  pi. 
  22) 
  

   Fig. 
  92 
  Orthothetes 
  hydraulicus 
  •; 
  Distinguishing 
  characters. 
  Small 
  

  

  size; 
  obtuse 
  cardinal 
  margins 
  with 
  hinge 
  line 
  shorter 
  than 
  greatest 
  

   width 
  of 
  shell; 
  uniformly 
  rounded 
  front; 
  strong 
  rounded, 
  sharply 
  de- 
  

  

  