﻿1008 
  FORTT-SEVEhTH 
  REPORT 
  ON 
  THE 
  STATE 
  MUSEUM. 
  

  

  Subgenus 
  Orthonomsea, 
  Hall. 
  1850. 
  

   (Plate 
  40, 
  figs. 
  34-37). 
  

  

  Large 
  shells, 
  having 
  the 
  contour 
  of 
  Zygospira, 
  but 
  with 
  ex- 
  

   tremely 
  fine 
  surface 
  plications. 
  

  

  (Type, 
  Orthonomcea 
  erratica, 
  Hall. 
  Hudson 
  River 
  group.) 
  

  

  Clintonella, 
  Hall. 
  1893. 
  

   (Plate 
  40, 
  figs. 
  38-44.) 
  

  

  Shells 
  usually 
  small, 
  suboval 
  in 
  outline 
  ; 
  valves 
  subequally 
  

   biconvex, 
  the 
  axis 
  of 
  greatest 
  convexity 
  being 
  oblique, 
  making 
  

   an 
  angle 
  of 
  about 
  fifty-five 
  degrees 
  with 
  the 
  vertical 
  axis 
  of 
  the 
  

   shell. 
  Pedicle-valve 
  with 
  a 
  small 
  umbo, 
  which 
  is 
  compressed 
  

   laterally, 
  the 
  apex 
  being 
  slightly 
  incurved. 
  The 
  cardinal 
  area 
  is 
  

   replaced 
  by 
  a 
  wide 
  triangular 
  delthyrium, 
  which 
  is 
  un 
  accom- 
  

   panied 
  by 
  any 
  trace 
  of 
  deltidial 
  plates. 
  The 
  medially 
  elevated 
  

   umbo 
  merges 
  anteriorly 
  into 
  a 
  sinus 
  which 
  makes 
  a 
  deep 
  flexure 
  

   at 
  the 
  margin 
  ; 
  it 
  bears 
  two 
  plications, 
  both 
  of 
  which 
  reach 
  the 
  

   Deak 
  ; 
  sometimes 
  a 
  trace 
  of 
  a 
  third 
  plication 
  may 
  be 
  seen. 
  The 
  

   lateral 
  slopes 
  bear 
  from 
  four 
  to 
  eight 
  radial 
  plications 
  of 
  

   smaller 
  size. 
  

  

  On 
  the 
  interior 
  the 
  teeth 
  are 
  prominent, 
  strongly 
  recurved 
  at 
  

   their 
  tips 
  and 
  supported 
  by 
  lamellae 
  which 
  terminate 
  abruptly. 
  

   The 
  lower 
  and 
  inner 
  margins 
  of 
  these 
  lamellae 
  are 
  thickened, 
  

   contracting 
  the 
  pedicle 
  cavity, 
  which 
  is, 
  consequently, 
  narrow 
  

   and 
  deep. 
  The 
  diductor 
  scars 
  are 
  of 
  moderate 
  size, 
  flabellate 
  in 
  

   outline 
  and 
  deeply 
  impressed 
  at 
  their 
  posterior 
  extremity. 
  They 
  

   are 
  crossed 
  by 
  traces 
  of 
  the 
  radial 
  surface 
  plications. 
  Between 
  

   them 
  lie 
  the 
  narrow 
  obovate 
  adductor 
  scars. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  beak 
  is 
  inconspicuous 
  ; 
  the 
  umbonal 
  

   region 
  depressed 
  for 
  about 
  one-third 
  the 
  length 
  of 
  the 
  shell, 
  

   thence 
  anteriorly 
  becoming 
  developed 
  into 
  a 
  median 
  fold. 
  The 
  

   greatest 
  convexity 
  of 
  the 
  valve 
  is 
  attained 
  in 
  front 
  of 
  the 
  center. 
  

   The 
  cardinal 
  margin 
  is 
  scarcely 
  thickened 
  ; 
  the 
  dental 
  sockets 
  

   quite 
  narrow. 
  The 
  hinge- 
  plate 
  consists 
  of 
  two 
  flattened 
  pro- 
  

   cesses, 
  inclined 
  toward 
  each 
  other 
  and 
  closely 
  approximate 
  along 
  

   their 
  inner 
  bases, 
  though 
  not 
  meeting 
  Each 
  process 
  is 
  divided 
  

   into 
  an 
  anterior 
  and 
  posterior 
  lobe, 
  the 
  latter 
  being 
  the 
  smaller 
  

   and 
  resting 
  upon 
  the 
  former. 
  These 
  anterior 
  lobes 
  are 
  narrow 
  

  

  66 
  

  

  