﻿Report 
  of 
  tee 
  State 
  Geologist. 
  1007 
  

  

  volution 
  of 
  the 
  ribbon 
  lies 
  just 
  within 
  the 
  margins 
  of 
  the 
  valves, 
  

   and 
  the 
  number 
  of 
  volutions 
  is 
  small. 
  The 
  spirals 
  have 
  their 
  

   bases 
  parallel 
  to 
  the 
  lateral 
  slopes 
  of 
  the 
  pedicle-valve 
  and 
  their 
  

   apices 
  directed 
  obliquely 
  toward 
  the 
  center 
  of 
  the 
  opposite 
  valve. 
  

   The 
  jugum 
  is 
  a 
  continuous 
  band, 
  variable 
  in 
  position 
  and 
  shape. 
  It 
  

   may 
  originate 
  on 
  the 
  posterior 
  or 
  anterior 
  limb 
  of 
  the 
  primary 
  

   lamellae, 
  or 
  be 
  placed 
  medially 
  ; 
  its 
  apex 
  is 
  always 
  angular 
  and 
  

   directed 
  anteriorly 
  and 
  the 
  lateral 
  curves 
  vary 
  in 
  length 
  and 
  

   degree 
  according 
  to 
  their 
  position 
  with 
  reference 
  to 
  the 
  spirals. 
  

  

  Type, 
  Zygospira 
  modesta, 
  (Say) 
  Hall. 
  Hudson 
  Ri^er 
  group. 
  

  

  Distribution. 
  Lower 
  — 
  Upper 
  Silurian. 
  

  

  Subgenus 
  Catazyga, 
  Hall. 
  1893. 
  

   (Plate 
  40. 
  figs. 
  26-33.) 
  

   Shells 
  rather 
  large, 
  subcircular 
  or 
  ovoid, 
  with 
  valves 
  more 
  

   convex 
  than 
  in 
  Ztoospika, 
  the 
  rotundity 
  of 
  the 
  pedicle-valve 
  

   obscuring 
  the 
  usual 
  prominence 
  of 
  the 
  umbo 
  in 
  that 
  genus. 
  Both 
  

   valves 
  bear 
  a 
  low 
  median 
  sinus, 
  while 
  the 
  external 
  surface, 
  instead 
  

   of 
  being 
  coarsely 
  plicated 
  as 
  in 
  Zygospira, 
  is 
  covered 
  with 
  a 
  

   great 
  number 
  of 
  fine 
  radiating 
  striae. 
  The 
  typical 
  external 
  expres- 
  

   sion 
  of 
  Zygospira. 
  is 
  thus 
  to 
  a 
  large 
  degree 
  lost. 
  On 
  the 
  interior 
  of 
  

   the 
  pedicle- 
  valve 
  the 
  muscular 
  impressions 
  are 
  well 
  defined 
  ; 
  the 
  

   pedicle-cavity 
  is 
  deep, 
  and 
  in 
  front 
  of 
  it 
  lies 
  a 
  more 
  deeply 
  exca- 
  

   vated, 
  short, 
  sharply 
  defined 
  and 
  longitudinally 
  striated 
  impres- 
  

   sion. 
  In 
  the 
  brachial 
  valve 
  is 
  a 
  broad 
  anterior 
  and 
  a 
  narrow, 
  

   elongate 
  posterior 
  pair 
  of 
  scars. 
  The 
  spirals 
  are 
  of 
  essentially 
  

   the 
  same 
  character 
  as 
  in 
  Z. 
  modesta, 
  though 
  

   the 
  form 
  of 
  the 
  cones 
  is 
  such 
  that 
  their 
  apices 
  

   converge 
  toward 
  the 
  median 
  line 
  in 
  a 
  plane 
  

   just 
  below 
  the 
  surface 
  of 
  the 
  brachial 
  valve. 
  

   The 
  jugum, 
  however, 
  differs; 
  it 
  is, 
  in 
  the 
  first 
  

   place, 
  persistently 
  posterior 
  in 
  its 
  position, 
  

   originating 
  as 
  in 
  Atrypa, 
  the 
  lateral 
  lamellae 
  otcat7zw^H^di^. 
  

   bending 
  downward 
  toward 
  the 
  bottom 
  of 
  the 
  "^(sp); 
  showing 
  the 
  

  

  ° 
  form 
  of 
  the 
  spiral 
  cones 
  

  

  brachial-valve 
  and 
  directed 
  forward 
  in 
  lines 
  and 
  jugum 
  as 
  viewed 
  

   which 
  are 
  parallel 
  for 
  a 
  short 
  distance. 
  Thence 
  y 
  l° 
  v 
  ™ 
  

   they 
  bend 
  inward 
  and 
  upward, 
  meeting 
  in 
  a 
  short 
  angle 
  in 
  the 
  

   space 
  just 
  behind 
  the 
  apices 
  of 
  the 
  spirals. 
  

  

  (Type, 
  Catazyga 
  JSeadi, 
  Billings 
  (sp.). 
  Lower 
  and 
  Middle 
  

   Silurian.) 
  

  

  65 
  

  

  