﻿1098 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  cardinal 
  line 
  may 
  be 
  regarded 
  as 
  extending 
  nearly 
  to 
  the 
  lateral 
  

   extremities 
  of 
  the 
  valves 
  ; 
  the 
  articulating 
  apparatus 
  consisting 
  

   of 
  a 
  pair 
  of 
  long 
  marginal 
  ridge-like 
  teeth 
  on 
  the 
  divergent 
  

   cardinal 
  slopes, 
  fitting 
  into 
  narrow 
  marginal 
  grooves 
  on 
  the 
  

   brachial 
  valve. 
  There 
  is 
  sometimes 
  a 
  trace 
  of 
  a 
  median 
  septum 
  

   over 
  the 
  pallial 
  region. 
  In 
  the 
  brachial 
  valve 
  is 
  a 
  small 
  callus, 
  

   boss 
  or 
  cardinal 
  process 
  lying 
  directly 
  beneath 
  the 
  apex. 
  Below 
  

   this 
  is 
  a 
  strong 
  median 
  septum, 
  which 
  increases 
  in 
  height 
  ante- 
  

   riorly 
  and 
  rises 
  to 
  an 
  acute, 
  anteriorly 
  directed 
  apex 
  at 
  about 
  two- 
  

   thirds 
  the 
  length 
  of 
  the 
  shell. 
  In 
  front 
  of 
  this 
  point 
  its 
  anterior 
  

   edge 
  is 
  concave, 
  the 
  septum 
  disappearing 
  not 
  far 
  within 
  the 
  

   margin 
  of 
  the 
  valve. 
  

  

  ]STo 
  traces 
  of 
  muscular 
  scars 
  have 
  been 
  observed 
  on 
  either 
  

   valve. 
  

  

  The 
  external 
  surface 
  of 
  the 
  valves 
  is 
  covered 
  by 
  a 
  coarse 
  net- 
  

   work 
  of 
  superficial 
  cells, 
  usually 
  hexagonal, 
  sometimes 
  circular 
  in 
  

   outline. 
  In 
  all 
  species 
  and 
  in 
  early 
  growth-stages 
  there 
  is 
  a 
  bare, 
  

   smooth 
  triangular 
  area 
  at 
  the 
  beak 
  of 
  the 
  pedicle-valve, 
  where 
  

   this 
  superficial 
  ornament 
  does 
  not 
  extend. 
  

  

  Type, 
  Eichwaldia 
  subtrigonalis, 
  Billings. 
  

  

  Distribution. 
  Lower 
  Silurian 
  — 
  Lower 
  Devonian. 
  

  

  Aulacorhynchus, 
  Dittmar. 
  1871. 
  

   (Plate 
  54, 
  figs. 
  23, 
  24.) 
  

  

  Shells 
  short, 
  transversely 
  elongate 
  or 
  alate 
  ; 
  extremities 
  often 
  

   rounded 
  ; 
  hinge-line 
  straight, 
  usually 
  making 
  the 
  greatest 
  width 
  

   of 
  the 
  shell. 
  Yalves 
  very 
  thin 
  and 
  fragile. 
  Pedicle-valve 
  slightly 
  

   convex, 
  with 
  traces 
  of 
  a 
  broad, 
  obscure 
  median 
  sinus; 
  brachial 
  

   valve 
  flat. 
  Surface 
  covered 
  with 
  numerous 
  regular 
  and 
  continu- 
  

   ous, 
  concentric 
  rounded 
  folds 
  or 
  ridges 
  which 
  are 
  separated 
  by 
  

   furrows 
  of 
  equal 
  width. 
  

  

  In 
  the 
  pedicle- 
  valve 
  the 
  character 
  of 
  the 
  articulating 
  processes 
  

   has 
  not 
  been 
  fully 
  ascertained. 
  There 
  appears, 
  however, 
  to 
  have 
  

   been 
  no 
  cardinal 
  area, 
  and 
  but 
  exceedingly 
  small 
  teeth, 
  judging 
  

   from 
  the 
  structure 
  of 
  the 
  brachial 
  valve. 
  Just 
  within 
  the 
  apex 
  of 
  

   the 
  valve, 
  which 
  is 
  closely 
  appressed 
  against 
  the 
  opposite 
  one, 
  

   begins 
  a 
  pair 
  of 
  divergent, 
  elevated 
  ridges, 
  which 
  extend 
  for 
  one- 
  

   third, 
  or 
  even 
  one-half 
  the 
  length 
  of 
  the 
  shell, 
  and 
  inclose 
  a 
  

   thickened 
  area 
  or 
  platform, 
  which 
  terminates 
  abruptly 
  in 
  a 
  trans- 
  

   verse 
  anterior 
  margin. 
  This 
  platform 
  is 
  the 
  seat 
  of 
  the 
  adductor 
  

  

  156 
  

  

  