﻿468 
  C. 
  0. 
  Dunbar 
  — 
  Rensselcerina, 
  a 
  New 
  Genus. 
  

  

  by 
  its 
  entirely 
  smooth 
  surface 
  and, 
  especially 
  on 
  the 
  interior, 
  

   by 
  the 
  incomplete 
  loop, 
  the 
  clearly 
  defined 
  double 
  dorsal 
  septa, 
  

   and 
  the 
  extremely 
  large 
  rostral 
  cavity 
  which 
  apparently 
  has 
  

   become 
  a 
  true 
  spondylium 
  and 
  is 
  supported 
  by 
  a 
  high 
  median 
  

   septum. 
  

  

  Rensselcerla 
  differs 
  in 
  its 
  uniformly 
  striate 
  surface, 
  in 
  its 
  

   wide, 
  strong, 
  and 
  recumbent 
  dental 
  lamellae 
  whose 
  outer 
  sides 
  

   rest 
  on 
  the 
  bottom 
  of 
  the 
  valve 
  and 
  which 
  unite 
  medially 
  to 
  

   make 
  a 
  broad 
  and 
  more 
  or 
  less 
  deep 
  rostral 
  cavity 
  ; 
  but 
  most 
  

   especially 
  in 
  its 
  simpler 
  brachidium, 
  which 
  is 
  without 
  the 
  

   median 
  vertical 
  lamella 
  of 
  the 
  new 
  genus. 
  

  

  Beachia 
  is 
  readily 
  distinguished 
  from 
  Rensselwrina 
  by 
  its 
  

   flat 
  ovate 
  shape, 
  its 
  erect 
  ventral 
  beak 
  which 
  exposes 
  the 
  delti- 
  

   dial 
  plates, 
  its 
  smooth 
  or 
  finely 
  striated 
  surface, 
  and 
  its 
  inverted 
  

   margins, 
  but 
  particularly 
  by 
  its 
  simpler 
  Rensseheria-like 
  

   loop, 
  with 
  a 
  median 
  posterior 
  spine 
  instead 
  of 
  an 
  anterior 
  ver- 
  

   tical 
  lamella. 
  

  

  Lissopleura 
  is 
  for 
  the 
  time 
  being 
  provisionally 
  placed 
  in 
  the 
  

   Centronellidse, 
  although 
  it 
  is 
  not 
  yet 
  clearly 
  understood. 
  At 
  

   any 
  rate, 
  it 
  differs 
  widely 
  from 
  the 
  new 
  genus 
  under 
  considera- 
  

   tion 
  in 
  its 
  rhynchonelloid 
  shape, 
  in 
  its 
  coarse 
  plications 
  which 
  

   have 
  no 
  interspaces 
  between 
  them, 
  and 
  in 
  its 
  high 
  dorsal 
  

   septum. 
  

  

  One 
  other 
  genus 
  needs 
  comparison. 
  Romingerina 
  is 
  based 
  

   on 
  a 
  single 
  species, 
  R. 
  julia, 
  which 
  is 
  a 
  very 
  small, 
  smooth, 
  

   lenticular 
  shell 
  from 
  the 
  Lower 
  Mississippian 
  of 
  Michigan. 
  

   The 
  only 
  character 
  which 
  seems 
  to 
  relate 
  this 
  genus 
  to 
  the 
  new 
  

   one 
  is 
  the 
  presence 
  of 
  a 
  median 
  vertical 
  lamella 
  on 
  the 
  loop. 
  

   However, 
  this 
  is 
  evidently 
  only 
  a 
  case 
  of 
  parallel 
  development. 
  

   The 
  lamella 
  in 
  Romingerina 
  is 
  described 
  by 
  Hall 
  and 
  Clarke 
  

   as 
  follows 
  :* 
  " 
  The 
  median 
  ridge 
  on 
  the 
  anterior 
  plate 
  of 
  the 
  

   brachidium 
  is 
  elevated 
  into 
  a 
  conspicuous 
  vertical 
  lamella, 
  ex- 
  

   tended 
  both 
  anteriorly 
  and 
  posteriorly, 
  being 
  in 
  fact 
  a 
  double 
  

   plate 
  produced 
  by 
  the 
  abrupt 
  deflection 
  of 
  each 
  lateral 
  branch 
  

   of 
  the 
  brachidium 
  near 
  the 
  median 
  line 
  ; 
  union 
  taking 
  place 
  

   along 
  the 
  upper 
  edge, 
  which 
  almost 
  reaches 
  the 
  inner 
  surface 
  

   of 
  the 
  pedicle 
  valve." 
  

  

  It 
  is 
  but 
  a 
  simple 
  step 
  to 
  the 
  development 
  of 
  this 
  type 
  of 
  

   brachidium 
  out 
  of 
  one 
  of 
  the 
  later 
  Centronellas. 
  Furthermore, 
  

   the 
  shell 
  of 
  Romingerina 
  has 
  a 
  smooth 
  surface, 
  its 
  beak 
  is 
  

   erect 
  enough 
  to 
  disclose 
  the 
  deltidial 
  plates, 
  and 
  it 
  frequently 
  

   shows 
  a 
  faint 
  fold 
  and 
  sinus 
  — 
  all 
  centronelloid 
  characters. 
  The 
  

   lateral 
  branches 
  of 
  the 
  loop 
  in 
  Centronella 
  are 
  also 
  sharply 
  de- 
  

   flected 
  before 
  meeting 
  medially, 
  their 
  deflected 
  edges 
  forming 
  

   a 
  slight 
  ridge. 
  A 
  mere 
  expansion 
  of 
  this 
  character 
  will 
  pro- 
  

   duce 
  essentially 
  the 
  loop 
  of 
  Romingerina. 
  

  

  *Hall 
  and 
  Clarke, 
  Pal. 
  N. 
  Y., 
  vol. 
  viii, 
  Pt. 
  2, 
  1894, 
  p. 
  271. 
  Winchell's 
  

   drawings 
  erroneously 
  show 
  the 
  crura 
  directed 
  dorsally. 
  

  

  