﻿138 
  

  

  T, 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TEIMEEELLTD^. 
  

  

  Points 
  in 
  which 
  the 
  Trimerellids 
  may 
  be 
  

   said 
  to 
  differ 
  from 
  the 
  Lingulids. 
  

  

  1. 
  Calcareous 
  shell-substance. 
  

  

  2. 
  Closed 
  at 
  the 
  hinge. 
  

  

  Dentary, 
  though 
  rudely. 
  

  

  Brachial 
  valve 
  with 
  a 
  cardinal 
  pro- 
  

  

  Points 
  in 
  which 
  the 
  Trimerellids 
  may 
  be 
  

   said 
  to 
  agree 
  with 
  the 
  Lingulids. 
  

  

  1. 
  Deltidium 
  and 
  its 
  accessories. 
  

  

  2. 
  Mode 
  of 
  attachment 
  of 
  the 
  pedicle 
  

  

  to 
  the 
  deltidium. 
  

  

  3. 
  Muscular 
  system 
  generally. 
  

  

  4. 
  Splanchnocoele 
  in 
  its 
  general 
  out- 
  

  

  line. 
  

  

  5. 
  The 
  possession 
  of 
  pleurocceles. 
  

  

  6. 
  General 
  arrangement 
  of 
  the 
  vascular 
  

  

  5. 
  Apophysary 
  system 
  generally 
  ele- 
  

  

  vated 
  platforms, 
  one 
  in 
  each 
  valve 
  ; 
  

   and 
  the 
  platforms 
  in 
  the 
  type 
  

   genus 
  doubly 
  vaulted. 
  

  

  6. 
  Absence 
  of 
  transmedial 
  muscles, 
  

  

  and 
  modifications 
  of 
  the 
  umbonal 
  

   muscle. 
  

  

  7. 
  Setal 
  band 
  not 
  carried 
  round 
  the 
  7. 
  The 
  combination 
  of 
  certain 
  parts 
  

   hinge. 
  or 
  organs, 
  producing 
  thereby 
  the 
  

  

  posterior 
  crescent. 
  

  

  The 
  differences 
  between 
  the 
  two 
  groups, 
  we 
  contend, 
  are 
  quite 
  

   sufficient 
  to 
  warrant 
  the 
  step 
  we 
  have 
  taken 
  in 
  making 
  of 
  each 
  a 
  

   distinct 
  family 
  ; 
  while, 
  on 
  the 
  other 
  hand, 
  their 
  points 
  of 
  agreement 
  

   are 
  so 
  numerous 
  as 
  to 
  show 
  that 
  they 
  are 
  intimately 
  related. 
  Obolus 
  

   is 
  placed 
  by 
  Dall 
  in 
  the 
  family 
  Lingulidce 
  ; 
  but 
  its 
  close 
  relationship 
  

   to 
  Discina 
  opposes 
  the 
  allocation. 
  This 
  relationship 
  alone 
  prevents 
  

   our 
  placing 
  the 
  Trimerellids 
  in 
  any 
  family 
  group 
  which 
  embraces 
  

   Obolus, 
  though 
  we 
  are 
  quite 
  ready 
  to 
  admit 
  that 
  both 
  belong 
  to 
  one 
  

   and 
  the 
  same 
  great 
  division 
  in 
  their 
  class. 
  

  

  At 
  first 
  sight 
  the 
  Trimerellids, 
  considering 
  their 
  general 
  external 
  

   resemblance 
  to 
  Uncites, 
  String 
  oceplicdus, 
  and 
  other 
  genera, 
  might 
  be 
  

   considered 
  to 
  belong 
  to 
  the 
  great 
  subclass, 
  Clistenterata; 
  but, 
  although 
  

   certain 
  parts 
  belonging 
  to 
  their 
  hinge 
  seem 
  to 
  favour 
  a 
  different 
  view, 
  

   it 
  cannot 
  but 
  be 
  admitted 
  that 
  the 
  consensus 
  of 
  their 
  characters 
  

   completely 
  establishes 
  them 
  as 
  Tretenterates*. 
  

  

  Y. 
  Geological 
  Range, 
  Chronogenesis, 
  and 
  Evolution 
  of 
  the 
  

   Family. 
  

  

  The 
  Trimerellids, 
  as 
  will 
  be 
  seen 
  hereafter, 
  characterize 
  two 
  consecu- 
  

   tive 
  geological 
  systems 
  — 
  the 
  Cambro-Silurian, 
  and 
  Silurian. 
  Neither 
  

   the 
  Cambrian 
  f 
  nor 
  Devonian 
  rocks 
  have 
  as 
  yet 
  yielded 
  any 
  repre- 
  

  

  * 
  Tretenterata 
  (open-gut) 
  and 
  Clistenterata 
  (closed-gut) 
  form 
  two 
  great 
  sub- 
  

   divisions 
  of 
  the 
  Palliobranchs 
  or 
  Brachiopoda. 
  The 
  names 
  Pleuropygia 
  and 
  

   Apygia, 
  proposed 
  some 
  years 
  ago 
  by 
  Bronn, 
  stand 
  for 
  the 
  same 
  groups. 
  The 
  

   latter, 
  being 
  a 
  negative 
  term, 
  is 
  totally 
  inadmissible 
  ; 
  while 
  the 
  former 
  not 
  only 
  

   prejudges 
  and 
  limits 
  the 
  position 
  of 
  an 
  important 
  organ 
  in 
  a 
  variable 
  group 
  of 
  

   shells, 
  but 
  it 
  seemB 
  likely 
  to 
  turn 
  out 
  to 
  be 
  no 
  more 
  than 
  of 
  sectional 
  value. 
  The 
  

   singular 
  aperture, 
  and 
  tube, 
  respectively 
  characterizing 
  the 
  hinge-plate 
  of 
  the 
  

   brachial 
  valve 
  of 
  Athyris 
  pectinifera, 
  and 
  A. 
  concentrica, 
  also 
  the 
  foramen 
  

   occurring, 
  in 
  addition 
  to 
  an 
  open 
  deltidium, 
  in 
  the 
  umbo 
  of 
  the 
  pedicle-valve 
  of 
  

   several 
  other 
  paleozoic 
  shells 
  apparently 
  belonging 
  to 
  the 
  Leptamids, 
  Spirife- 
  

   rids, 
  Obolids, 
  and 
  Bhynchonellids, 
  may 
  be 
  regarded 
  as 
  indicating, 
  in 
  such 
  cases, 
  

   the 
  posterior 
  position 
  of 
  the 
  intestinal 
  outlet. 
  — 
  W. 
  K. 
  

  

  t 
  We 
  include 
  in 
  the 
  Cambrian 
  system 
  the 
  formations 
  below 
  the 
  Arenigs 
  — 
  

   namely, 
  the 
  Tremadocs, 
  Ffestiniogs, 
  Menevians, 
  and 
  Longmynds 
  : 
  as 
  such 
  it 
  an- 
  

   swers 
  to 
  the 
  Primordial 
  group 
  of 
  Barrande. 
  The 
  Arenig, 
  Llandeilo, 
  and 
  Bala 
  

  

  