﻿1006 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  Zygospira, 
  Hall. 
  1862. 
  

   Synonym, 
  Anazyga, 
  Davidson, 
  1882. 
  

   (Plate 
  40, 
  figs. 
  13-25.) 
  

   Shells 
  usually 
  small. 
  Outline 
  subcircular 
  or 
  transversely 
  oval. 
  

   Contour 
  subplano-convex. 
  Surface 
  sharply 
  plicate. 
  Pedicle- 
  

   valve 
  with 
  a 
  median 
  plicated 
  ridge. 
  Umbo 
  narrow 
  and 
  promi- 
  

   nent; 
  beak 
  acute 
  and 
  incurved. 
  Foramen 
  elongate, 
  rarely 
  

   apical, 
  enclosed 
  by 
  the 
  deltidial 
  plates. 
  Hinge-line 
  long 
  and 
  

   straight 
  ; 
  cardinal 
  extremities 
  rounded. 
  A 
  distinct 
  false 
  area 
  is 
  

   formed 
  by 
  a 
  pair 
  of 
  ridges 
  diverging 
  from 
  the 
  beak 
  toward 
  the 
  

  

  Fig. 
  393. 
  

  

  Fig. 
  395. 
  Fig. 
  396. 
  

  

  Preparations 
  of 
  Zygospira 
  modesta, 
  (Say) 
  Hall; 
  showing 
  the 
  variation 
  in 
  position 
  of 
  the 
  jugum. 
  

  

  cardinal 
  extremities. 
  On 
  the 
  interior 
  the 
  teeth 
  are 
  moderately 
  

   well 
  developed 
  and 
  unsupported 
  by 
  dental 
  lamellae. 
  

  

  The 
  brachial 
  valve 
  is 
  depressed 
  convex 
  in 
  the 
  umbonal 
  region 
  

   and 
  bears 
  a 
  more 
  or 
  less 
  conspicuous 
  median 
  sinus. 
  The 
  hinge- 
  

   plate 
  consists 
  of 
  two 
  broad, 
  stout 
  processes, 
  diverging 
  outwardly, 
  

   grooved 
  on 
  their 
  summits, 
  and 
  separated 
  from 
  each 
  other 
  by 
  a 
  

   narrow, 
  sharp 
  cleft. 
  They 
  form 
  both 
  the 
  socket 
  walls 
  and 
  crural 
  

   bases, 
  and 
  are 
  supported 
  by 
  a 
  low 
  median 
  ridge. 
  Muscular 
  

   impressions 
  obscure 
  in 
  the 
  typical 
  species. 
  

  

  The 
  crura 
  are 
  short 
  and 
  straight 
  at 
  their 
  union 
  with 
  the 
  

   primary 
  lamellae, 
  making 
  a 
  rectangular 
  curve. 
  The 
  first 
  half- 
  

  

  64 
  

  

  