﻿1016 
  jBortt- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  Amphiclinodonta, 
  Bittner. 
  1890. 
  

  

  Shells 
  exteriorly 
  like 
  Ahphiclina; 
  interiorly 
  articulation 
  is 
  aided 
  

   by 
  interlocking 
  denticulate 
  callous 
  ridges 
  and 
  tubercles 
  lying 
  

  

  Fig. 
  414. 
  — 
  Amphiclinodonta 
  Zugmayeri, 
  Bittner. 
  (Bittner). 
  

  

  within 
  the 
  margins 
  of 
  the 
  valves. 
  Structure 
  of 
  the 
  brachidium 
  as 
  

   in 
  Amphiclina. 
  

  

  Type, 
  Amphiclinodonta 
  Zugmayeri, 
  Bittner. 
  

  

  Distribution. 
  Triassic. 
  

  

  Rhynchonella, 
  Fischer 
  de 
  Waldheim. 
  1809. 
  

  

  The 
  number 
  of 
  palaeozoic 
  species 
  which 
  have 
  been 
  commonly 
  

   referred 
  to 
  this 
  genus, 
  and, 
  consequently, 
  regarded 
  as 
  congeneric 
  

   with 
  the 
  Russian 
  upper 
  Jurassic 
  R. 
  loxia, 
  Fischer, 
  the 
  type-species, 
  

   is 
  very 
  great. 
  To 
  the 
  most 
  conservative 
  student 
  such 
  an 
  agglom- 
  

   eration, 
  presenting 
  every 
  variety 
  of 
  external 
  configuration, 
  must 
  

   seem 
  more 
  like 
  a 
  hap-hazard 
  and 
  conventional 
  association 
  than 
  

   a 
  natural 
  group. 
  But 
  it 
  is, 
  nevertheless, 
  evident 
  that 
  features 
  of 
  

   internal 
  structure, 
  upon 
  the 
  variations 
  in 
  which 
  generic 
  dis- 
  

   tinctions 
  are 
  usually 
  based, 
  are 
  here 
  most 
  persistent. 
  The 
  

   crura, 
  hinge 
  apparatus 
  and 
  deltidial 
  structure 
  of 
  R. 
  loxia 
  are 
  

   features 
  which 
  were 
  attained 
  and 
  became 
  fixed 
  in 
  the 
  Silurian 
  

   period 
  ; 
  the 
  extreme 
  pyramidal 
  contour 
  of 
  that 
  species, 
  its 
  smooth 
  

   surface 
  with 
  few 
  and 
  faint 
  marginal 
  plications, 
  is 
  not, 
  however, 
  

   except 
  in 
  rare 
  instances, 
  reproduced 
  among 
  the 
  palaeozoic 
  forms. 
  

   What 
  is 
  thus 
  true 
  of 
  the 
  predecessors 
  of 
  R. 
  loxia 
  is 
  also, 
  to 
  a 
  

   large 
  degree 
  at 
  least, 
  true 
  of 
  its 
  living 
  descendants. 
  

  

  From 
  a 
  careful 
  study 
  of 
  the 
  structure 
  of 
  the 
  ancient 
  Rhynchon- 
  

   ellas 
  it 
  has 
  become 
  apparent 
  that 
  slight 
  variations 
  from 
  the 
  type 
  

   of 
  interior 
  possessed 
  by 
  R. 
  loxia 
  are 
  frequently 
  of 
  marked 
  con- 
  

  

  