﻿828 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL- 
  MUSEUM. 
  vol.xx. 
  

  

  HEMIPECTEN 
  Adams 
  and 
  Reeve, 
  1849. 
  

   TTemiperleii 
  Vekrill, 
  Trans. 
  Conn. 
  Acad., 
  X, 
  pp. 
  (iO, 
  89, 
  91, 
  1897. 
  

  

  Type. 
  — 
  Ilemipecten 
  forbesianus 
  Adams 
  and 
  lieeve. 
  

  

  This 
  group 
  includes 
  species 
  with 
  thin, 
  irreguhir 
  shells 
  attached 
  by 
  

   the 
  right 
  valve, 
  like 
  Hinvites, 
  but 
  the 
  attachment 
  is 
  effected 
  mainly 
  

   by 
  a 
  ])ermanent 
  modified 
  byssus. 
  The 
  posterior 
  auricles 
  are 
  nearly 
  

   obsolete. 
  The 
  byssal 
  notch 
  becomes 
  irregular 
  and 
  nearly 
  luclosed, 
  as 
  

   in 
  Anomia. 
  

  

  PSEUDAMUSIUM 
  H. 
  and 
  A. 
  Adams, 
  l858. 
  

  

  Pseudamus'mm 
  (pars) 
  H. 
  and 
  A. 
  Adams, 
  1858 
  (after 
  Klein). 
  — 
  Ciienu, 
  1862. 
  — 
  Sto- 
  

   LICZKA, 
  1871.— 
  ZiTTEL, 
  1881. 
  — 
  FiscHER, 
  1887. 
  — 
  Dall, 
  1886 
  (pars). 
  — 
  Veuuill, 
  

   Trans. 
  Conn. 
  Acad., 
  X, 
  pp. 
  60, 
  90, 
  92, 
  pi. 
  xvii, 
  figs. 
  8, 
  8a, 
  1897 
  (restricted). 
  

  

  Type. 
  — 
  rHendamusium 
  exotieum 
  (Chemnitz, 
  Lamarck). 
  

  

  The 
  typical 
  species 
  of 
  this 
  group 
  have 
  nearly 
  smooth, 
  round, 
  sym- 
  

   metrical, 
  closed 
  shells 
  with 
  well 
  defined, 
  small, 
  straight, 
  obtuse-angled 
  

   auricles. 
  Tlie 
  valves 
  are 
  nearly 
  equal 
  and 
  have 
  nearly 
  simple, 
  

   even 
  margins. 
  The 
  external 
  sculpture 
  consists 
  of 
  small 
  radial 
  stri.e 
  or 
  

   riblots, 
  without 
  strong 
  angular 
  ribs 
  and 
  grooves, 
  and 
  it 
  may 
  differ 
  

   on 
  the 
  two 
  valves. 
  Some 
  of 
  the 
  species 
  show 
  the 
  fine 
  divergent 
  

   "camptonectes 
  sculpture" 
  on 
  one 
  or 
  both 
  valves, 
  especially 
  when 
  

   young. 
  The 
  margin 
  is 
  uot 
  scalloped, 
  or 
  but 
  faintly 
  so, 
  and 
  there 
  are 
  

   no 
  definite 
  internal 
  ribs. 
  The 
  hinge-plate 
  has 
  usually 
  but 
  one 
  longi- 
  

   tudinal 
  fold 
  on 
  each 
  end 
  which 
  is 
  feeble 
  and 
  nearly 
  parallel 
  with 
  the 
  

   marginal 
  ligamental 
  groove 
  and 
  is 
  usually 
  cross-lined. 
  The 
  byssal 
  

   not(;li 
  is 
  small 
  and 
  the 
  pectinidial 
  teeth 
  vary 
  from 
  one 
  up 
  to 
  five 
  in 
  

   number, 
  or 
  sometimes 
  may 
  be 
  lacking. 
  

  

  CAMPTONECTES 
  Meek, 
  1864. 
  

  

  Camptonectef< 
  Meek, 
  1864. 
  — 
  Stoliczka, 
  1871. 
  — 
  Zittkl, 
  1881 
  (type, 
  arenatns 
  Gold- 
  

   fnss).— 
  Vekhill, 
  Trans. 
  Conn. 
  Acad., 
  X, 
  pp. 
  02, 
  90, 
  91, 
  1897. 
  

  

  Type. 
  — 
  Camptonectes 
  leiui 
  (Sowerby). 
  

  

  Shell 
  subovate, 
  plain, 
  not 
  corrugated, 
  and 
  without 
  strong 
  radial 
  

   ribs; 
  margin 
  nearly 
  plain. 
  Valves 
  subeiiual. 
  Auricles 
  unequal; 
  byssal 
  

   notch 
  well 
  developed. 
  Surface 
  of 
  the 
  shell 
  covered 
  with 
  fine, 
  obliquely 
  

   divergent, 
  curved, 
  crenulated 
  or 
  vermiculated 
  riblets 
  with 
  intervening, 
  

   narrow, 
  i)unctate 
  grooves. 
  

  

  The 
  curious 
  vermiculated 
  sculpture 
  is 
  not 
  peculiar 
  to 
  this 
  division, 
  

   but 
  is 
  more 
  or 
  less 
  obvious 
  on 
  the 
  shells 
  of 
  some 
  species 
  of 
  rseuda- 
  

   ■mnsium, 
  and 
  on 
  species 
  of 
  several 
  other 
  groups, 
  both 
  with 
  and 
  without 
  

   radial 
  ribs. 
  It 
  is 
  a 
  structural 
  feature 
  that 
  runs 
  obliquely 
  across 
  the 
  

   ribs 
  and 
  grooves. 
  Most 
  of 
  the 
  species 
  are 
  Mesozoic 
  fossils. 
  

  

  The 
  recent 
  Pecten 
  striatus 
  and 
  P. 
  tigrinns 
  Lamarck, 
  of 
  Europe, 
  

   apparently 
  belong 
  to 
  this 
  group, 
  and 
  F. 
  testce 
  might 
  also 
  well 
  be 
  

   referred 
  to 
  it. 
  The 
  latter 
  is 
  one 
  of 
  the 
  types 
  of 
  Falliolum 
  Avhich 
  

   might 
  well 
  be 
  regarded 
  as 
  a 
  section 
  of 
  this 
  genus. 
  

  

  