﻿Report 
  of 
  the 
  State 
  Geologist. 
  1017 
  

  

  tinuance, 
  and 
  we 
  must, 
  therefore, 
  be 
  prepared 
  for 
  closer 
  dis- 
  

   criminations 
  in 
  this 
  great 
  group 
  of 
  species 
  than 
  have 
  elsewhere 
  

   been 
  necessary 
  or 
  advisable, 
  and 
  to 
  emphasize 
  such 
  of 
  these 
  

   deviations 
  from 
  this 
  stable 
  line 
  of 
  development, 
  as 
  are 
  justified 
  

   by 
  their 
  persistence 
  and 
  the 
  convenience 
  of 
  classification. 
  

  

  Rhynchonella, 
  sensu 
  stricto. 
  

   (Plate 
  42, 
  figs. 
  1-3.) 
  

  

  Subpyramidal 
  shells 
  having 
  the 
  margins 
  of 
  the 
  valves 
  sinuous 
  

   or 
  angulated. 
  Pedicle-valve 
  with 
  a 
  median 
  sinus 
  beginning 
  in 
  

   front 
  of 
  the 
  convex 
  umbo 
  and, 
  in 
  the 
  type-species, 
  becoming 
  

   broad 
  and 
  deep, 
  producing 
  a 
  prominent 
  linguiform 
  extension 
  at 
  

   the 
  anterior 
  margin. 
  Brachial 
  valve 
  convex 
  in 
  the 
  umbonal 
  

   region 
  and 
  developing 
  anteriorly 
  a 
  prominent 
  median 
  fold. 
  

   Surface 
  of 
  both 
  valves 
  more 
  or 
  less 
  plicated, 
  often 
  accompanied 
  

   (as 
  in 
  the 
  type)- 
  by 
  Une 
  concentric 
  lines 
  of 
  ornament. 
  The 
  apex 
  

   of 
  the 
  pedicle-valve 
  is 
  but 
  slightly 
  incurved 
  and 
  exposes 
  a 
  cir- 
  

   cular 
  or 
  elongate-oval 
  foramen 
  enclosed 
  by 
  the 
  deltidial 
  plates 
  

   beneath, 
  and 
  above 
  by 
  the 
  substance 
  of 
  the 
  valves. 
  Here 
  is 
  a 
  

   narrow 
  pseudo-area 
  defined 
  by 
  oblique 
  cardinal 
  ridges 
  diverging 
  

   from 
  the 
  beak. 
  On 
  the 
  interior 
  the 
  teeth 
  are 
  well 
  developed 
  and 
  

   are 
  supported 
  by 
  lamella 
  which 
  rest 
  on 
  the 
  bottom 
  of 
  the 
  valve 
  

   near 
  the 
  beak, 
  but 
  are 
  free 
  anteriorly. 
  The 
  muscular 
  area 
  con- 
  

   sists 
  of 
  a 
  moderately 
  deep 
  oval 
  scar 
  extending 
  one- 
  third 
  the 
  

   length 
  of 
  the 
  valve, 
  and 
  composed 
  of 
  two 
  large 
  diductors 
  com- 
  

   pletely 
  enclosing 
  small 
  central 
  adductors. 
  The 
  posterior 
  surface 
  

   about 
  the 
  muscular 
  area 
  is 
  pitted 
  with 
  ovarian 
  markings. 
  

  

  In 
  the 
  brachial 
  valve 
  there 
  is 
  no 
  cardinal 
  process 
  ; 
  the 
  crural 
  

   plates 
  are 
  simple, 
  divergent, 
  somewhat 
  expanded 
  on 
  the 
  upper 
  

   surface 
  but 
  not 
  conjoined 
  except 
  where 
  they 
  converge 
  beneath 
  

   the 
  beak 
  and 
  meet 
  the 
  median 
  septum, 
  which 
  extends 
  for 
  about 
  

   one-half 
  the 
  length 
  of 
  the 
  valve. 
  The 
  crura 
  are 
  long 
  and 
  curved 
  

   upward 
  toward 
  the 
  opposite 
  valve. 
  Muscular 
  area 
  elongate 
  sub- 
  

   quadrate, 
  with 
  small 
  posterior 
  and 
  large 
  anterior 
  adductor 
  scars 
  

   Shell-structure 
  fibrous. 
  

  

  T} 
  r 
  pe, 
  Rhynchonella 
  loxia, 
  Fischer 
  de 
  Waldheim. 
  

  

  Distribution. 
  Jurassic. 
  Cretaceous. 
  

   128 
  75 
  

  

  