﻿376 
  Prof. 
  T. 
  R. 
  Jones 
  on 
  the 
  

  

  vol. 
  xvi. 
  1855, 
  pp. 
  90 
  and 
  174, 
  pi. 
  v. 
  fig. 
  23, 
  and 
  pi. 
  vi. 
  

   figs. 
  28-31; 
  and 
  ser. 
  3, 
  vol. 
  xvi. 
  1865, 
  p. 
  419), 
  in 
  one 
  or 
  

   other 
  of 
  its 
  modifications, 
  as 
  described 
  in 
  1865. 
  See 
  also 
  

   Proc. 
  Geol. 
  Assoc. 
  1869, 
  " 
  Pal. 
  Biv. 
  Entom.," 
  pp. 
  8, 
  10, 
  and 
  

   13, 
  figs. 
  I. 
  and 
  II. 
  

  

  Primitia 
  simplex 
  is 
  the 
  relatively 
  short-backed 
  and 
  P. 
  mun- 
  

   dula 
  the 
  long-backed 
  of 
  the 
  two 
  well-known 
  and 
  most 
  

   common 
  of 
  the 
  Silurian 
  forms 
  of 
  minute 
  Ostracods. 
  

  

  Fig. 
  1. 
  Fig. 
  2. 
  

  

  Fig. 
  1. 
  — 
  Outline 
  of 
  Primitia 
  simplex. 
  

   Fig. 
  2. 
  — 
  Outline 
  of 
  Primitia 
  mundula. 
  

   Both 
  magnified 
  S 
  diameters. 
  

  

  PI. 
  XVI. 
  fig. 
  1 
  (on 
  no. 
  15 
  specimen) 
  is 
  an 
  internal 
  cast, 
  

   oblong, 
  with 
  marginal 
  rim 
  (broken) 
  and 
  a 
  neat 
  sulcus, 
  with- 
  

   out 
  thickened 
  edges. 
  The 
  matrix 
  is 
  a 
  ferruginous 
  hard 
  shale, 
  

   with 
  small 
  gasteropods 
  (obscure) 
  and 
  some 
  plant-remains. 
  

  

  Fig. 
  2 
  (on 
  no. 
  5) 
  is 
  a 
  very 
  neat 
  valve, 
  nearly 
  semicircular 
  

   along 
  the 
  free 
  margins, 
  with 
  a 
  narrow 
  and 
  weak 
  sulcus, 
  and 
  

   ornamented 
  with 
  a 
  delicate 
  reticulation 
  (fig. 
  2 
  b). 
  

  

  Fig. 
  4 
  (specimen 
  no. 
  22) 
  is 
  a 
  neat 
  suboblong 
  cast 
  of 
  

   P. 
  mundula, 
  but 
  with 
  the 
  sulcus 
  more 
  forward, 
  and 
  less 
  

   developed 
  at 
  its 
  edges, 
  than 
  in 
  the 
  typical 
  form. 
  Other 
  Pri- 
  

   mitice 
  of 
  somewhat 
  similar 
  pattern 
  occur 
  with 
  it, 
  also 
  small 
  

   gasteropods 
  and 
  plant-remains. 
  

  

  Fig. 
  5 
  (specimen 
  no. 
  10) 
  is 
  a 
  short 
  oblong 
  variety 
  of 
  P. 
  

   mundula 
  with 
  well-developed 
  sulcus 
  (partly 
  damaged) 
  and 
  an 
  

   extremely 
  delicate 
  reticulate 
  ornament, 
  visible 
  on 
  some 
  part 
  

   of 
  the 
  shell. 
  With 
  plant-remains. 
  

  

  Fig. 
  6 
  (specimen 
  no. 
  18) 
  is 
  a 
  typical 
  P. 
  mundula, 
  rather 
  

   fuller 
  in 
  the 
  ventral 
  curve 
  than 
  fig. 
  29 
  a, 
  pi. 
  vi. 
  1855, 
  and 
  

   with 
  feeble 
  tubercles 
  at 
  the 
  sides 
  of 
  the 
  sulcus. 
  The 
  reticu- 
  

   late 
  ornament 
  of 
  the 
  valve's 
  surface 
  is 
  shown 
  in 
  fig. 
  6 
  b. 
  

   Many 
  other 
  simple 
  Primitice 
  accompany 
  it, 
  also 
  fish-remains. 
  

  

  Fig. 
  7 
  (specimen 
  no. 
  12). 
  This 
  cast 
  does 
  not 
  differ 
  

   much 
  in 
  shape 
  from 
  figs. 
  1, 
  2, 
  4, 
  and 
  6, 
  except 
  in 
  the 
  

   fuller 
  outline 
  and 
  extent; 
  of 
  the 
  posterior 
  moiety 
  ; 
  but 
  the 
  

   sulcus 
  is 
  more 
  forward 
  and 
  its 
  front 
  edge 
  is 
  modified 
  into 
  a 
  

   short 
  swelling, 
  going 
  off, 
  with 
  a 
  sinuous 
  margin, 
  into 
  an 
  

   antero-dorsal 
  depression. 
  This 
  condition 
  is 
  possibly 
  shown 
  to 
  

   a 
  slight 
  extent 
  in 
  fig. 
  6 
  a 
  ; 
  and, 
  if 
  so, 
  the 
  gradation 
  to 
  P. 
  

   mundula 
  is 
  sufficiently 
  strong 
  to 
  prevent 
  us 
  giving 
  fig. 
  7 
  a 
  

  

  