﻿Report 
  of 
  the 
  State 
  Geologtst. 
  985 
  

  

  Rhynchospira, 
  Hall. 
  1859. 
  

   (Plate 
  36. 
  figs. 
  8-12.) 
  

  

  Shells 
  elongate, 
  retziiform; 
  hinge-line 
  short 
  and 
  curved. 
  Umbo 
  

   of 
  the 
  pedicle-valve 
  incurved, 
  usually 
  concealing 
  the 
  deltarium 
  ; 
  

   apex 
  truncated 
  by 
  a 
  circular 
  foramen. 
  Cardinal 
  slopes 
  gradual, 
  

   scarcely 
  excavated, 
  not 
  forming 
  a 
  false 
  area. 
  Deltarium 
  triangular 
  

   and 
  flat 
  or 
  incurved 
  ; 
  its 
  lateral 
  margins 
  are 
  sharply 
  defined, 
  and 
  

   its 
  surface 
  traversed 
  bv 
  a 
  longitudinal 
  median 
  ridge, 
  which 
  is 
  the 
  

   line 
  of 
  solid 
  coalescence 
  of 
  the 
  constituent 
  plates. 
  The 
  umbonal 
  

   cavity 
  does 
  not 
  contain 
  the 
  split 
  deltidial 
  tube 
  which 
  is 
  present 
  

   in 
  the 
  genera 
  Retzia, 
  Httstedia., 
  etc. 
  The 
  teeth 
  are 
  small 
  and 
  

   well 
  defined, 
  and 
  are 
  not 
  supported 
  by 
  dental 
  plates. 
  Muscular 
  

   impressions 
  very 
  obscure. 
  

  

  In 
  the 
  brachial 
  valve 
  the 
  hinge-plate 
  has 
  the 
  general 
  form 
  of 
  

   that 
  in 
  Teematospika, 
  but 
  is 
  much 
  less 
  elevated. 
  Its 
  posterior 
  

   extension 
  is 
  slight, 
  extending 
  but 
  a 
  short 
  distance 
  beyond 
  the 
  

   hinge 
  ; 
  it 
  consists 
  of 
  two 
  parts, 
  a 
  lower, 
  which 
  is 
  closely 
  appressed 
  

   agdinst 
  the 
  umbo, 
  and 
  deeply 
  divided 
  by 
  a 
  median 
  cleft 
  ; 
  and 
  an 
  

   upper, 
  which 
  is 
  larger, 
  conspicuously 
  elevated 
  and 
  divided 
  medially 
  

   only 
  at 
  its 
  margin, 
  though 
  the 
  groove 
  extends 
  forward 
  to 
  the 
  

   middle 
  of 
  the 
  plate. 
  The 
  anterior 
  portion 
  is 
  deeply 
  concave 
  and 
  

   produced 
  into 
  two 
  flat 
  lobes 
  which 
  form 
  the 
  crural 
  bases. 
  The 
  

   entire 
  plate 
  rests 
  on 
  stout 
  supports 
  which 
  diverge 
  at 
  the 
  bottom, 
  

   leaving 
  a 
  triangular 
  cavity 
  beneath, 
  in 
  which 
  there 
  is 
  a 
  short, 
  

   sometimes 
  obscure 
  median 
  septum. 
  The 
  spirals 
  make 
  from 
  six 
  

   to 
  nine 
  volutions, 
  the 
  primary 
  lamellae 
  being 
  nar- 
  

   row 
  and 
  not 
  greatly 
  incurved. 
  The 
  jugum 
  is 
  

   situated 
  behind 
  the 
  middle 
  of 
  these 
  lamellae 
  and 
  

   is 
  simple 
  in 
  its 
  structure 
  ; 
  its 
  lateral 
  branches 
  

   narrow 
  just 
  above 
  their 
  origin, 
  with 
  a 
  gentle 
  poste- 
  

   rior 
  inclination, 
  then 
  broaden 
  and 
  meet 
  at 
  a 
  little 
  j 
  U 
  ram 
  of 
  5 
  siTn^o- 
  

   more 
  than 
  one 
  half 
  the 
  distance 
  across 
  the 
  base 
  of 
  spiraformosa, 
  Haii. 
  

   the 
  coils, 
  forming 
  a 
  broad, 
  short, 
  roof-shaped 
  process, 
  which 
  is 
  

   directed 
  posteriorly 
  and 
  terminates 
  in 
  an 
  oblique 
  edge. 
  

  

  The 
  external 
  surface 
  is 
  radially 
  plicate, 
  the 
  plications 
  being 
  

  

  simple. 
  In 
  young 
  shells 
  there 
  is 
  a 
  median 
  sinus 
  on 
  both 
  valves, 
  

  

  but 
  as 
  growth 
  advances, 
  that 
  of 
  the 
  brachial 
  valve 
  develops 
  into 
  

  

  a 
  low 
  fold. 
  Both 
  fold 
  and 
  sinus 
  bear 
  a 
  number 
  of 
  small, 
  intercalary 
  

  

  124 
  43 
  

  

  