﻿THE 
  GENERA 
  OF 
  THE 
  BRACHIOPODA. 
  

  

  ii. 
  

   bkachiopoda 
  aeticulata. 
  

  

  (CONTINUED) 
  

  

  Spirifer, 
  Sowerby. 
  1815. 
  

   (Plates 
  23-29.) 
  

  

  Synonyms 
  ; 
  Trigonotreta, 
  Kcenig, 
  1825 
  ; 
  Fusella 
  and 
  Brachy- 
  

   thyris, 
  McCoy, 
  1841. 
  

  

  Shells 
  transvt 
  rsely 
  elongate, 
  rarely 
  produced 
  axially 
  ; 
  with 
  or 
  

   without 
  median 
  fold 
  and 
  sinus. 
  Hinge-line 
  straight, 
  usually 
  form- 
  

   ing 
  the 
  greatest 
  diameter 
  of 
  the 
  shell, 
  but 
  in 
  some 
  of 
  the 
  subdi- 
  

   visions 
  of 
  the 
  genus, 
  short 
  and 
  inconspicuous. 
  Cardinal 
  extrem- 
  

   ities 
  alate, 
  acuminate 
  or 
  rounded. 
  

  

  Surface 
  covered 
  with 
  granulations, 
  striae, 
  plications 
  or 
  costa, 
  

   variously 
  grouped 
  on 
  the 
  lateral 
  slopes 
  and 
  which 
  may 
  be 
  pres- 
  

   ent 
  or 
  absent 
  on 
  the 
  median 
  fold 
  and 
  sinus 
  ; 
  these 
  are 
  crossed 
  by 
  

   concentric 
  growth-lines 
  which 
  may 
  take 
  the 
  form 
  of 
  varices 
  or 
  

   expanded 
  lamellae, 
  or 
  be 
  modified 
  into 
  fimbriae 
  of 
  simple 
  or 
  com- 
  

   pound 
  spines. 
  In 
  the 
  subgenera 
  Martinia 
  and 
  Martiniopsis 
  the 
  

   surface 
  is 
  smooth 
  except 
  for 
  the 
  concentric 
  striae. 
  Shell 
  substance 
  

   fibrous, 
  generally 
  impumtate 
  ; 
  in 
  the 
  smooth 
  species 
  the 
  epider- 
  

   mal 
  layer 
  is 
  minutely 
  pitted. 
  

  

  The 
  pedicle- 
  valve 
  has 
  the 
  umbo 
  more 
  or 
  less 
  elevated 
  over 
  the 
  

   hinge-line, 
  the 
  apex 
  acute, 
  erect 
  or 
  incurved. 
  The 
  cardinal 
  slopes 
  

   show 
  a 
  slight 
  tendency 
  to 
  concavity 
  or 
  excavation, 
  and 
  the 
  

   median 
  portion 
  of 
  the 
  valve 
  is 
  more 
  or 
  less 
  strongly 
  depressed 
  by 
  

   a 
  sinus. 
  The 
  cardinal 
  area 
  is 
  broad, 
  flat 
  or 
  incurved 
  and 
  its 
  

   surface 
  is 
  transversely 
  striated 
  ; 
  the 
  inner 
  shell- 
  layers 
  bear 
  a 
  

   series 
  of 
  longitudinal 
  or 
  vertical 
  canals 
  at 
  whose 
  marginal 
  

   extremities 
  the 
  fibrous 
  tissue 
  is 
  sometimes 
  produced 
  into 
  a 
  row 
  

   of 
  denticles, 
  corresponding 
  to 
  a 
  row 
  of 
  pits 
  on 
  the 
  opposite 
  valve 
  

   119 
  3 
  

  

  