﻿C. 
  O. 
  Dunbar 
  — 
  Rensselcerina, 
  a 
  New 
  Genus. 
  467 
  

  

  The 
  ventral 
  interior 
  differs 
  from 
  that 
  of 
  Rensselceria 
  in 
  

   having 
  very 
  small 
  vertical 
  dental 
  plates 
  that 
  do 
  not 
  attain 
  the 
  

   bottom 
  of 
  the 
  valve, 
  while 
  the 
  pedicle 
  or 
  rostral 
  cavity 
  between 
  

   them 
  is 
  very 
  deep 
  and 
  narrow. 
  The 
  muscle 
  scar 
  is 
  limited 
  to 
  

   the 
  posterior 
  half 
  of 
  the 
  shell. 
  The 
  diductor 
  scars 
  are 
  elon- 
  

   gate 
  and 
  slender. 
  Between 
  their 
  more 
  deeply 
  impressed 
  pos- 
  

   terior 
  portions 
  are 
  embraced 
  the 
  narrow 
  imprints 
  of 
  the 
  

   adductor 
  muscles. 
  The 
  dorsal 
  interior 
  has 
  a 
  thick 
  and 
  con- 
  

   spicuous 
  triangular 
  hinge-plate, 
  supported 
  by 
  two 
  thickened 
  

   crural 
  lamellae. 
  This 
  plate 
  is 
  highly 
  variable. 
  Most 
  commonly 
  

   it 
  is 
  thickened 
  medially 
  into 
  a 
  slight 
  triangular 
  elevation, 
  the 
  

   actual 
  cardinal 
  process 
  (see 
  Plate 
  II, 
  figs. 
  14-18), 
  on 
  which 
  are 
  

   the 
  scars 
  of 
  the 
  cardinal 
  muscles; 
  but 
  all 
  gradations 
  occur 
  

   from 
  such 
  an 
  elevation 
  to 
  a 
  decided 
  triangular 
  pit 
  (Plate 
  II, 
  

   ligs. 
  14, 
  15) 
  for 
  the 
  insertion 
  of 
  these 
  muscles, 
  or 
  to 
  the 
  other 
  

   extreme 
  where 
  a 
  distinct 
  median 
  process 
  arises 
  (Plate 
  II, 
  tig. 
  

   18), 
  at 
  the 
  sides 
  of 
  which 
  lie 
  the 
  muscle 
  scars. 
  This 
  tendency 
  

   toward 
  an 
  elevation 
  of 
  the 
  center 
  of 
  the 
  hinge-plate 
  is 
  in 
  

   marked 
  contrast 
  with 
  Rensselceria, 
  wherein 
  the 
  plate 
  is 
  primi- 
  

   tively 
  flat 
  but 
  in 
  later 
  forms 
  always 
  has 
  a 
  depressed 
  cardinal 
  

   pit. 
  As 
  in 
  most 
  Lower 
  Devonian 
  terebratulids, 
  the 
  hinge- 
  

   plate 
  is 
  traversed 
  by 
  a 
  well 
  defined 
  visceral 
  foramen, 
  which, 
  

   beginning 
  dorsad 
  of 
  the 
  anterior 
  edge 
  of 
  the 
  plate, 
  emerges 
  

   on 
  its 
  surface 
  just 
  below 
  the 
  beak. 
  The 
  adductor 
  muscle 
  

   scars 
  are 
  narrow, 
  separate, 
  and 
  distinctly 
  impressed. 
  

  

  The 
  brachium 
  is 
  distinctive 
  and 
  may 
  be 
  described 
  as 
  follows 
  : 
  

   The 
  thin, 
  slightly 
  divergent, 
  descending 
  lamellae 
  are 
  broad 
  and 
  

   stand 
  nearly 
  vertical 
  dorso-ventrally 
  as 
  they 
  leave 
  the 
  wide 
  and 
  

   conspicuous 
  crural 
  processes 
  ; 
  then, 
  diverging 
  more 
  rapidly, 
  

   the 
  lamellae 
  become 
  narrower 
  and 
  rotate 
  into 
  a 
  horizontal 
  

   position 
  as 
  the 
  loop 
  attains 
  its 
  greatest 
  width, 
  and 
  by 
  a 
  regular 
  

   curvature 
  begins 
  to 
  converge 
  (Plate 
  II, 
  figs. 
  1, 
  3). 
  Approach- 
  

   ing 
  the 
  median 
  line, 
  just 
  in 
  front 
  of 
  the 
  middle 
  of 
  the 
  shell, 
  

   the 
  loop 
  broadens 
  into 
  a 
  spoon-like 
  plate, 
  drawn 
  out 
  anteriorly 
  

   into 
  a 
  long 
  acumination 
  which 
  curves 
  gently 
  downward. 
  The 
  

   edge 
  of 
  the 
  loop, 
  which 
  was 
  ventral 
  when 
  it 
  left 
  the 
  crurae, 
  is 
  

   now 
  anterior 
  and 
  forms 
  the 
  outer 
  margin 
  of 
  this 
  plate, 
  dipping 
  

   outward 
  at 
  about 
  40° 
  below 
  the 
  horizontal. 
  From 
  its 
  concave 
  

   ventral 
  surface 
  arises 
  a 
  vertical 
  median 
  lamella 
  (Plate 
  II, 
  fig. 
  2) 
  

   which 
  extends 
  forward 
  beyond 
  this 
  plate, 
  just 
  in 
  front 
  of 
  

   which 
  it 
  reaches 
  its 
  greatest 
  height. 
  It 
  continues 
  anteriorly 
  to 
  

   near 
  the 
  front 
  of 
  the 
  shell, 
  where 
  it 
  approaches 
  the 
  ventral 
  

   valve. 
  

  

  Relationships. 
  — 
  This 
  genus 
  is 
  one 
  of 
  the 
  several 
  divergent 
  

   lines 
  or 
  phyla 
  of 
  the 
  Centronellidse. 
  While 
  in 
  general 
  the 
  ex- 
  

   ternal 
  aspect 
  resembles 
  most 
  Amphigenia 
  and 
  Rensselwria, 
  it 
  

   presents 
  essential 
  differences. 
  Amphigenia 
  is 
  at 
  once 
  set 
  off 
  

  

  