﻿Report 
  of 
  the 
  State 
  Geologist. 
  

  

  813 
  

  

  latest 
  and 
  only 
  ^existing 
  brachiopod 
  which 
  retains 
  a 
  trne 
  deltidium 
  

   at 
  maturity. 
  Daring 
  the 
  cephalula 
  stage 
  of 
  the 
  embryo, 
  before 
  

   the 
  inversion 
  of 
  j 
  the 
  mantle 
  lobes 
  to 
  enclose 
  the 
  head, 
  two 
  shell- 
  

   plates 
  begin 
  to 
  form, 
  one 
  on 
  the 
  inner 
  side 
  of 
  the 
  dorsal 
  mantle 
  

   lobe, 
  the 
  other 
  directly 
  opposite 
  to 
  it 
  on 
  the 
  onter 
  surface 
  of 
  that 
  

   portion 
  of 
  the 
  body 
  which 
  subsequently 
  becomes 
  the 
  pedicle. 
  

  

  Fig. 
  252. 
  

  

  Fig. 
  254. 
  

  

  Fig. 
  257. 
  

  

  Fig. 
  253. 
  

  

  Fig. 
  255. 
  

  

  Fig. 
  256. 
  

  

  Fig 
  253. 
  

  

  Fig. 
  252. 
  

   Fig. 
  253. 
  

  

  Fig. 
  254. 
  

  

  Thecidea 
  (Lacazelld) 
  Mediterranea. 
  

   Cephalula, 
  dorsal 
  side; 
  showing 
  below, 
  the 
  cephalic 
  segment 
  with 
  eye 
  spots, 
  and 
  on 
  

  

  the 
  upper 
  segment 
  the 
  dorsal 
  shell-plate. 
  

   Dorso- 
  ventral 
  longitudinal 
  section 
  of 
  cephalula; 
  below 
  i» 
  the 
  cephalic 
  segment, 
  at 
  the 
  

   right 
  the 
  dorsal 
  mantle 
  lobe, 
  the 
  thick 
  line 
  en 
  its 
  inner 
  margin 
  representing 
  the 
  

   beginning 
  of 
  the 
  dorsal 
  valve, 
  and 
  the 
  similar 
  line 
  on 
  the 
  adjoining 
  side 
  of 
  th 
  

   body 
  the 
  incipient 
  deltidium. 
  

   A. 
  later 
  growth 
  stage, 
  in 
  which 
  the 
  mantle 
  lobes 
  have 
  turned 
  downward. 
  The 
  body 
  

   shell-plate 
  is 
  seen 
  in 
  the 
  upper 
  part 
  of 
  the 
  figure. 
  

   Fig. 
  255. 
  Dorso-ventral 
  longitudinal 
  section 
  of 
  the 
  preceding; 
  showing 
  the 
  inversion 
  of 
  both 
  

   mantle 
  lobes. 
  The 
  relations 
  of 
  the 
  dorsal 
  and 
  body 
  (deltidium) 
  plates 
  are 
  indicated 
  

   by 
  the 
  heavy 
  lines 
  at 
  the 
  right. 
  The 
  ventral 
  plate 
  is 
  also 
  seen 
  on 
  the 
  lobe 
  at 
  the 
  

   left. 
  

   Fig. 
  256. 
  Profile 
  of 
  a 
  very 
  young 
  Leptcena 
  rJwmboidalis, 
  oriented 
  to 
  correspond 
  with 
  the 
  fore- 
  

   going 
  figures. 
  

   Figs. 
  257, 
  258. 
  Views 
  of 
  adult 
  Thecidea 
  mediterranea 
  similarly 
  placed. 
  

  

  (Beecher, 
  figs. 
  252-255, 
  adapted 
  from 
  Kowjxevski.) 
  

  

  In 
  this 
  condition 
  of 
  growth 
  the 
  ventral 
  lobe 
  of 
  the 
  mantle 
  is 
  but 
  

   slightly 
  developed 
  and 
  bears 
  no 
  shell-plates. 
  These 
  features 
  are 
  

   seen 
  in 
  the 
  accompanying 
  figure 
  of 
  a 
  longitudinal 
  section 
  of 
  such 
  

   an 
  embryo. 
  In 
  the 
  directly 
  following 
  growth-stage 
  the 
  reversion 
  

   of 
  the 
  mantle 
  lobes 
  has 
  taken 
  place 
  ; 
  the 
  shell-plate 
  before 
  on 
  the 
  

   inner 
  surface 
  of 
  the 
  dorsal 
  lobe 
  is 
  now 
  on 
  its 
  outer 
  surface, 
  and 
  

   assumes 
  the 
  normal 
  position 
  of 
  the 
  dorsal 
  or 
  brachial 
  valve. 
  A 
  

  

  