﻿826 
  PROCEEDIXaS 
  of 
  the 
  national 
  museum. 
  vol. 
  XX. 
  

  

  PECTEN 
  Muller, 
  1776. 
  

  

  I'ecten 
  (ist 
  section) 
  Klein, 
  1753 
  -f 
  Vola. 
  

  

  Pecten 
  Mullek, 
  Prod. 
  Zoiil. 
  Dan., 
  1776 
  {pam). 
  — 
  DaCosta, 
  1778. 
  — 
  Boi.ten, 
  1798 
  

  

  (restricted). 
  — 
  Cuvier, 
  1798. 
  — 
  Lamakck, 
  Syst., 
  ISOl. 
  — 
  Verrill, 
  Trans. 
  Conn. 
  

  

  Acad., 
  , 
  pp. 
  56, 
  89, 
  91, 
  1897. 
  

   Janira 
  Schumacher, 
  1817.— 
  Dall, 
  1886 
  (pars). 
  — 
  Fischer, 
  1887. 
  

   /'o/rt 
  H. 
  and 
  A. 
  Adams 
  (after 
  Klein), 
  18.58. 
  — 
  Stoliczka, 
  Menu 
  Geoloin-. 
  Snrvey 
  of 
  

  

  India, 
  Crctateous 
  Pelecypod 
  Fauna, 
  III, 
  p. 
  426, 
  1871. 
  — 
  Zittel, 
  ISSI. 
  

   I'ola 
  + 
  .Janira 
  Chenu, 
  1862. 
  

  

  Tyjye. 
  — 
  Pecten 
  mnximus 
  (LinnjTeus). 
  

  

  Since 
  Bolten, 
  in 
  1798, 
  definitely 
  restricted 
  the 
  name 
  Pecten 
  to 
  this 
  

   group, 
  his 
  restriction 
  has 
  precedence 
  over 
  that 
  of 
  Schumacher. 
  

  

  The 
  shells 
  are 
  generally 
  large 
  and 
  heavy, 
  and 
  the 
  valves 
  are 
  very 
  

   unequal, 
  even 
  when 
  very 
  young. 
  The 
  right 
  valve 
  is 
  strongly 
  convex 
  

   with 
  a 
  large 
  and 
  much 
  incurved 
  umbo 
  and 
  beak, 
  while 
  the 
  left 
  valve 
  

   is 
  flat 
  or 
  even 
  concave. 
  It 
  is 
  usually 
  smaller 
  than 
  the 
  right, 
  and 
  shuts 
  

   closely 
  inside 
  of 
  its 
  scalloped 
  margin, 
  and 
  its 
  umbo 
  is 
  nearly 
  or 
  quite 
  

   obsolete. 
  The 
  auricles 
  are 
  of 
  moderate 
  si/e 
  and 
  not 
  oblique, 
  and 
  in 
  the 
  

   right 
  valve 
  they 
  are 
  strongly 
  convex 
  or 
  excurved 
  in 
  the 
  middle. 
  This 
  

   valve 
  has 
  a 
  sinuous, 
  excurved 
  byssal 
  notch, 
  with 
  obsolete 
  pectinidial 
  

   teeth. 
  The 
  surface 
  of 
  both 
  valves 
  has 
  strong 
  radial 
  ribs 
  interlocking 
  

   at 
  the 
  margin. 
  Internally 
  there 
  are 
  angular, 
  thickened, 
  and 
  fluted 
  

   radial 
  ribs 
  oi)posite 
  the 
  external 
  grooves; 
  these 
  ribs 
  become 
  more 
  

   prominent 
  and 
  bicarinate 
  or 
  fluted 
  near 
  the 
  margins. 
  

  

  AMUSIUM 
  Bolten, 
  1798. 
  

  

  Amusiitm 
  Bolten, 
  1798. 
  — 
  Muhlfeldt, 
  1811. 
  — 
  Schuuacher, 
  1817.— 
  Woodward, 
  

   1866.- 
  Dall, 
  1886.— 
  Verrill, 
  Trans. 
  Conn. 
  Acad., 
  X, 
  pp. 
  57, 
  90, 
  92, 
  1897. 
  

  

  Amiissium 
  H. 
  and 
  A. 
  Adams, 
  1858 
  (pars). 
  — 
  Stoliczka, 
  Mem. 
  Geolog. 
  Survey 
  of 
  

   India, 
  Cretaceous 
  Pelecypod 
  Fauna, 
  III, 
  p. 
  426, 
  1871. 
  — 
  Fi.sciier, 
  1887.— 
  

   Zittel, 
  1881. 
  

  

  rienromctia 
  Swain, 
  1840.— 
  Chenu, 
  1862. 
  

  

  Tyjw. 
  — 
  Amusium 
  pleuronectes 
  (Linnaeus). 
  

  

  In 
  this 
  very 
  distinct 
  genus 
  the 
  shell 
  is 
  round, 
  thin, 
  nearly 
  smooth, 
  

   and 
  strongly 
  compressed. 
  The 
  surface 
  is 
  often 
  polished, 
  sometimes 
  

   lightly 
  radially 
  striated, 
  never 
  strongly 
  ribbed. 
  The 
  margins 
  are 
  sim- 
  

   ple 
  and 
  thin. 
  The 
  valves 
  may 
  be 
  a 
  little 
  unequal 
  in 
  convexity 
  and 
  

   usually 
  ditter 
  in 
  color 
  and 
  somewhat 
  in 
  sculpture. 
  The 
  valves 
  come 
  

   together 
  ventrally, 
  but 
  usually 
  gape 
  at 
  both 
  ends. 
  The 
  auricles 
  are 
  

   small, 
  symmetrical, 
  nearly 
  equilateral, 
  often 
  with 
  lateral 
  crurtv; 
  the 
  

   byssal 
  notch 
  is 
  small 
  or 
  absent, 
  pectinidial 
  teeth 
  nearly 
  or 
  quite 
  abor- 
  

   tive. 
  The 
  adult 
  probably 
  has 
  no 
  byssus. 
  Ilinge-plate 
  simple. 
  Interior 
  

   of 
  valves 
  strengthened 
  by 
  a 
  number 
  of 
  raised 
  divergent 
  ribs, 
  or 
  lira-, 
  

   independent 
  of 
  any 
  external 
  sculjpture. 
  

  

  