﻿56 
  (J. 
  E. 
  Beecher 
  — 
  Development 
  of 
  Bilobites. 
  

  

  figure 
  b. 
  B. 
  Vemeuilianus, 
  figure 
  d, 
  diverges 
  at 
  this 
  point, 
  

   progressively 
  increasing 
  its 
  variation 
  from 
  the 
  normal 
  direct 
  

   growth, 
  as 
  exemplified 
  in 
  B. 
  bilobus, 
  figure 
  e. 
  B. 
  various, 
  

   figure 
  e 
  y 
  passes 
  through 
  all 
  the 
  bilobus 
  stages, 
  and 
  culminates 
  

   in 
  larger 
  individuals, 
  with 
  less 
  pronounced 
  lobation 
  of 
  the 
  shell. 
  

   The 
  direct 
  line 
  of 
  development, 
  or 
  the 
  epacme, 
  is 
  repre- 
  

   sented 
  by 
  B. 
  bilobus, 
  and 
  it 
  is 
  significant 
  that 
  this 
  form 
  also 
  

   has 
  the 
  greatest 
  geological 
  and 
  geographical 
  distribution. 
  

   Next, 
  the 
  divergent 
  and 
  indirect 
  line, 
  or 
  acmic 
  group, 
  typified 
  

   by 
  B. 
  Vemeuilianus 
  and 
  B. 
  acutilobus, 
  is 
  also 
  widely 
  dis- 
  

   tributed, 
  but 
  less 
  so 
  than 
  the 
  first. 
  Finally, 
  the 
  paracmic, 
  or 
  

   geratologous 
  form, 
  B. 
  various, 
  culminated 
  and 
  disappeared 
  

   within 
  very 
  narrow 
  time 
  and 
  regional 
  limits. 
  

  

  Tale 
  Museum, 
  New 
  Haven, 
  Conn. 
  

  

  EXPLANATION 
  OF 
  PLATE 
  I. 
  

  

  Bilobites 
  acutilobus, 
  Ringueberg. 
  

   Figure 
  1. 
  — 
  Outline 
  of 
  specimen 
  from 
  Niagara 
  Group, 
  Lockport, 
  N. 
  Y. 
  x 
  4. 
  

  

  Bilobites 
  Vemeuilianus, 
  Lindstrom. 
  

  

  Figure 
  2. 
  — 
  Common 
  elongate 
  form 
  from 
  Upper 
  Silurian, 
  Gotland, 
  Sweden, 
  x 
  4. 
  

  

  Bilobites 
  various, 
  Conrad. 
  

  

  Figure 
  3. 
  — 
  Dorsal 
  view 
  of 
  youugest 
  individual 
  observed; 
  showing 
  inception 
  of 
  

   radiating 
  striae 
  and 
  concealment 
  of 
  hinge 
  areas, 
  x 
  18. 
  

  

  Figure 
  4. 
  — 
  Profile 
  of 
  same 
  ; 
  showing 
  depth 
  and 
  extent 
  of 
  both 
  valves, 
  x 
  1 
  8. 
  

  

  Figure 
  5. 
  — 
  Hinge 
  view 
  of 
  preceding, 
  x 
  18. 
  

  

  Figure 
  6. 
  — 
  Dorsal 
  side 
  of 
  specimen; 
  showing 
  beginning 
  of 
  anterior 
  marginal 
  

   sinus, 
  x 
  18. 
  

  

  Figure 
  7. 
  — 
  Profile 
  of 
  same, 
  x 
  18. 
  

  

  Figure 
  8. 
  — 
  Posterior 
  view 
  of 
  same, 
  x 
  18. 
  

  

  Figure 
  9. 
  — 
  Dorsal 
  view 
  of 
  specimen, 
  figure 
  15, 
  showing 
  concealment 
  of 
  ventral 
  

   area, 
  x 
  9. 
  

  

  Figure 
  10. 
  — 
  Ventral 
  view 
  of 
  same 
  ; 
  showing 
  dorsal 
  area 
  x 
  9. 
  Compare 
  this 
  with 
  

   dorsal 
  view 
  of 
  larger 
  specimen, 
  figure 
  21, 
  in 
  series. 
  

  

  Figures 
  11-26. 
  — 
  Series 
  of 
  specimens; 
  seen 
  from 
  dorsal 
  side; 
  exhibiting 
  ob- 
  

   served 
  stages 
  of 
  growth, 
  variation 
  and 
  development 
  of 
  hinge, 
  

   hinge 
  area, 
  and 
  marginal 
  sinus. 
  x 
  4. 
  

  

  Figure 
  27. 
  — 
  Interior 
  of 
  ventral 
  valve 
  ; 
  showing 
  teeth, 
  muscular 
  impressions, 
  

   minute 
  concave 
  plate 
  in 
  apex 
  of 
  fissure, 
  and 
  arrangement 
  of 
  

   punctas 
  between 
  nodes 
  and 
  ribs, 
  x 
  6. 
  Lower 
  Helderberg 
  group. 
  

   Albany 
  County, 
  N. 
  Y. 
  

  

  Bilobites 
  bilobus, 
  Linne. 
  

  

  Figure 
  28. 
  — 
  Outline 
  ; 
  showing 
  characteristic 
  form 
  of 
  this 
  species 
  as 
  occurring 
  in 
  

   Upper 
  Silurian 
  of 
  Gotland, 
  Sweden. 
  

  

  