﻿C. 
  E. 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  53 
  

  

  grown 
  and 
  fully 
  developed 
  specimens 
  of 
  Bilobites 
  various, 
  

   Conrad, 
  sp., 
  can 
  still 
  be 
  picked 
  up 
  in 
  considerable 
  numbers 
  in 
  

   the 
  soil 
  formed 
  of 
  the 
  decomposed 
  limestones. 
  The 
  species, 
  

   however, 
  is 
  not 
  so 
  abundant 
  as 
  formerly. 
  Professor 
  James 
  

   Hall 
  is 
  authority 
  for 
  the 
  statement 
  (Pal. 
  IN. 
  Y., 
  vol. 
  iii, 
  p. 
  493), 
  

   that 
  forty 
  thousand 
  individuals 
  were 
  collected 
  between 
  1843 
  

   and 
  1858, 
  and 
  about 
  four 
  thousand 
  in 
  the 
  four 
  following 
  years. 
  

   The 
  young 
  specimens 
  have 
  been 
  obtained 
  only 
  by 
  carefully 
  

   examining 
  the 
  decomposed 
  surfaces 
  of 
  the 
  limestones, 
  and 
  by 
  

   treating 
  with 
  hydrochloric 
  acid 
  slabs 
  of 
  rock 
  in 
  which 
  the 
  

   fossils 
  are 
  replaced 
  by 
  silica. 
  After 
  considerable 
  labor 
  and 
  

   search, 
  about 
  a 
  thousand 
  individuals 
  have 
  been 
  obtained. 
  

   From 
  this 
  number, 
  it 
  has 
  been 
  possible 
  to 
  select 
  a 
  series 
  of 
  

   over 
  forty 
  specimens, 
  showing 
  stages 
  of 
  growth 
  ranging 
  from 
  

   shells 
  a 
  little 
  less 
  than 
  one-half 
  a 
  millimeter 
  in 
  length 
  to 
  a 
  

   length 
  of 
  nine 
  millimeters 
  ; 
  thus 
  representing 
  the 
  development 
  

   between 
  these 
  limits 
  by 
  almost 
  insensible 
  gradations. 
  

  

  Developme?ital 
  Changes 
  in 
  Bilobites 
  various. 
  

  

  In 
  the 
  youngest 
  specimens 
  yet 
  detected, 
  measuring 
  -49 
  mm 
  in 
  

   length, 
  and 
  semi-elliptical 
  in 
  outline, 
  the 
  dorsal 
  valve 
  is 
  longer 
  

   than 
  the 
  ventral 
  ; 
  the 
  hinge 
  is 
  equal 
  to 
  the 
  greatest 
  width 
  of 
  

   the 
  shell 
  ; 
  both 
  areas 
  are 
  high, 
  subequal, 
  and 
  perforate 
  by 
  a 
  

   triangular 
  fissure 
  in 
  each 
  valve. 
  In 
  rare 
  instances, 
  the 
  pedicle 
  

   covering, 
  or 
  pseudo-deltidium, 
  is 
  retained 
  in 
  young 
  shells. 
  

   Figure 
  1 
  of 
  the 
  ventral 
  area, 
  shows 
  the 
  fissure 
  and 
  pedicle 
  

   covering, 
  with 
  the 
  foramen 
  at 
  the 
  

   apex 
  of 
  the 
  beak. 
  The 
  covering 
  is 
  

   soon 
  absorbed 
  or 
  abraded 
  during 
  sub- 
  

   sequent 
  growth, 
  and 
  the 
  pedicle 
  then 
  

   emerged 
  through 
  the 
  fissure 
  below. 
  

   None 
  of 
  these 
  characters 
  obtain 
  in 
  

  

  B. 
  various, 
  ventral 
  area, 
  x 
  25. 
  t 
  ] 
  ie 
  nea 
  logic 
  Or 
  ephebolic 
  stages, 
  

  

  which 
  are 
  represented 
  by 
  a 
  cordate, 
  bilobed 
  shell 
  ; 
  dorsal 
  valve 
  

   shorter 
  than 
  the 
  ventral 
  ; 
  hinge 
  line 
  much 
  shorter 
  than 
  the 
  

   width 
  of 
  the 
  shell, 
  and 
  an 
  inconspicuous 
  dorsal 
  area 
  without 
  a 
  

   fissure. 
  

  

  The 
  series 
  of 
  outlines, 
  figures 
  11 
  to 
  26, 
  drawn 
  to 
  the 
  same 
  

   scale, 
  illustrate 
  both 
  the 
  important 
  changes 
  which 
  take 
  place 
  in 
  

   the 
  general 
  form, 
  and 
  the 
  corresponding 
  increase 
  in 
  size 
  from 
  

   stage 
  to 
  stage. 
  The 
  rounded 
  frontal 
  margin 
  of 
  figures 
  11 
  and 
  12, 
  

   becomes 
  straight 
  in 
  figure 
  13, 
  and 
  in 
  figure 
  14 
  a 
  gentle 
  sinus 
  is 
  

   apparent, 
  which 
  is 
  pronounced 
  in 
  figure 
  15, 
  and 
  thereafter 
  is 
  

   the 
  conspicuous 
  character 
  of 
  the 
  entire 
  shell 
  up 
  to 
  the 
  ephe- 
  

   bolic 
  stage 
  represented 
  by 
  figure 
  23. 
  Figures 
  24 
  and 
  25 
  show 
  

  

  