﻿C. 
  E. 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  51 
  

  

  Art. 
  VII. 
  — 
  Development 
  of 
  Bilobites 
  ; 
  bv 
  Charles 
  E. 
  

   Beecher, 
  Ph.D. 
  (With 
  Plate 
  L) 
  

  

  The 
  Linnean 
  species 
  so 
  well 
  known 
  under 
  the 
  name 
  of 
  

   Orthis 
  biloba, 
  and 
  so 
  widely 
  distributed 
  in 
  the 
  Silurian 
  rocks 
  

   of 
  the 
  world, 
  reju'esents 
  one 
  of 
  the 
  very 
  distinct 
  members 
  into 
  

   which 
  the 
  Orthis 
  group 
  is 
  now 
  divided. 
  It 
  is 
  much 
  removed 
  

   from 
  ordinary 
  Orthis 
  in 
  general 
  external 
  features, 
  and 
  only 
  by 
  

   means 
  of 
  developmental 
  characters 
  is 
  it 
  possible 
  to 
  arrive 
  at 
  

   any 
  idea 
  of 
  its 
  genetic 
  history. 
  

  

  After 
  having 
  been 
  referred 
  to 
  various 
  genera, 
  including 
  

   Anomia, 
  Terebratula, 
  Delthyris, 
  and 
  Spirifer, 
  by 
  different 
  

   authors 
  prior 
  to 
  1848, 
  Davidson* 
  first 
  showed 
  conclusively, 
  

   from 
  a 
  study 
  of 
  the 
  internal 
  characters, 
  that 
  the 
  true 
  relations 
  

   were 
  with 
  the 
  genus 
  Orthis. 
  Its 
  position 
  has 
  since 
  remained 
  

   unchallenged, 
  and 
  subsequent 
  investigation 
  has 
  not 
  brought 
  

   forth 
  any 
  new 
  characters, 
  nor 
  invalidated 
  the 
  results 
  obtained 
  

   by 
  Davidson. 
  The 
  additional 
  observations 
  here 
  made 
  concern- 
  

   ing 
  the 
  development 
  of 
  the 
  shell, 
  while 
  adding 
  to 
  our 
  knowl- 
  

   edge 
  of 
  the 
  species, 
  merely 
  serve 
  to 
  bind 
  more 
  closely 
  this 
  form 
  

   to 
  the 
  group 
  having 
  the 
  broad 
  designation 
  of 
  Orthis. 
  Prof. 
  

   King 
  in 
  1850f 
  proposed 
  the 
  genus 
  Diccelosia 
  for 
  this 
  species, 
  

   on 
  account 
  of 
  its 
  characteristic 
  form, 
  and 
  authors 
  disposed 
  to 
  

   divide 
  Orthis 
  have 
  recognized 
  this 
  name. 
  Since 
  then, 
  it 
  has 
  

   been 
  shown 
  that 
  Linne 
  gave 
  the 
  generic 
  term 
  Bilobites 
  to 
  the 
  

   type 
  species 
  of 
  King's 
  genus, 
  and 
  this 
  name 
  is 
  now 
  generally 
  

   adopted 
  with 
  the 
  rank 
  of 
  a 
  subgenus. 
  The 
  validity 
  of 
  the 
  

   specific 
  names 
  applied 
  to 
  variations 
  from 
  the 
  typical 
  form 
  is 
  

   not 
  of 
  much 
  moment 
  in 
  this 
  place, 
  although 
  the 
  geologic 
  his- 
  

   tory 
  and 
  interpretation 
  of 
  these 
  differences 
  are 
  of 
  considerable 
  

   interest. 
  Two 
  well-defined 
  varieties 
  or 
  species 
  are 
  recognized 
  

   in 
  Sweden, 
  and 
  are 
  represented 
  in 
  outline 
  by 
  figures 
  2 
  and 
  28, 
  

   Plate 
  I. 
  The 
  prevailing 
  form 
  in 
  the 
  Wenlock 
  shales 
  at 
  

   Dudley, 
  England, 
  agrees 
  with 
  figure 
  28, 
  and 
  also 
  represents 
  the 
  

   ordinary 
  form 
  from 
  the 
  Niagara 
  Group 
  of 
  Indiana 
  and 
  New 
  

   York. 
  Each 
  locality, 
  however, 
  presents 
  minor 
  differences, 
  

   mainly 
  of 
  local 
  interest, 
  and 
  seldom 
  of 
  varietal 
  importance. 
  

   In 
  Western 
  New 
  York, 
  besides 
  the 
  ordinary 
  form 
  with 
  both 
  

   valves 
  convex 
  there 
  is 
  found 
  an 
  arcuate, 
  deeply 
  bilobed 
  variety, 
  

   agreeing 
  with 
  the 
  extreme 
  of 
  the 
  Swedish 
  B. 
  bilobus, 
  var. 
  

   V 
  trneuilianus 
  Lm., 
  represented 
  in 
  figure 
  2. 
  The 
  lobes 
  of 
  

   the 
  New 
  York 
  variety 
  are 
  commonly 
  more 
  divergent, 
  as 
  shown 
  

  

  *Bull. 
  Soc. 
  G-eol. 
  France. 
  2d 
  ser., 
  vol. 
  v. 
  p. 
  321, 
  t. 
  3, 
  fig. 
  18, 
  1848. 
  

   f 
  Monograph. 
  Permian 
  fossils, 
  page 
  106, 
  1850. 
  

  

  