ME.  HAKCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
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shell ; inclining  forwards  and  inwards,  they  go  to  be  attached  to  the  sides  of  the  cen- 
tral ridge  of  the  dorsal  valve,  about  one-thnd  the  length  of  the  shell  from  the  anterior 
margin.  The  dorsal  extremities  are  compressed,  and  have  then  sides  in  contact. 
The  posterior  occlusors  are  rather  stouter  and  much  shorter  than  the  anterior  pair, 
and  go  directly  from  valve  to  valve,  parallel  to  each  other,  one  a little  on  each  side  of 
the  longitudmal  central  line.  They  run  internal  to  the  anterior  occlusors,  and  are  just 
sufficiently  apart  from  each  other  to  allow  the  passage  of  the  alimentary  tube,  which 
rises  up  between  the  anterior  pair.  The  ventral  extremities  of  the  posterior  occlusors 
are  placed  a little  in  advance  of  the  corresponding  terminations  of  the  anterior  pair, 
and  the  dorsal  extremities  of  the  former  are  situated  a little  behind  those  of  the  latter. 
The  dorsal  ends  of  the  posterior  occlusors  are  partially  encircled  by  the  gastro-parietal 
bands. 
These  two  pans  of  muscles  are  mainly  instrumental  in  closing  the  valves,  and  there 
can  be  little  doubt  that  they  homologically  represent  the  occlusors  of  the  articulated 
Brachiopods.  There  are  here,  however,  four  distinct  muscles ; in  Waldlieimia  we  have 
seen  that  the  two  pairs  are  blended  at  then  ventral  extremities.  Both  pairs  in  Lingula 
are  attached  to  a transverse  fold  of  the  anterior  wall  of  the  body,  which  is  also  the  case 
with  the  occlusors  in  the  articulated  species ; and  in  them,  and  in  Lingula,  the  oesopha- 
gus is  similarly  related  to  these  muscles.  In  the  latter  species  the  anterior  occlusors 
are  inclined,  and  in  this  respect  resemble  the  closing  muscles  of  the  Terehratulidce,  and 
particularly  the  anterior  pair,  which  are  the  more  inclined  of  the  two.  The  posterior 
occlusors  are  peculiar  in  going  direct  from  valve  to  valve ; but  that  they  are  the  homo- 
logues  of  the  so-named  muscles  in  Waldlieimia,  appears  proved  by  their  relation  to  the 
gastro-parietal  bands,  which  is  the  same  in  both  cases. 
The  divaricator,  though  forming  a single  mass,  is  really  two  muscles  combined.  It  is 
stout  and  short,  and  is  situated  at  the  posterior  extremity  of  the  perivisceral  chamber, 
passing  directly  between  the  valves,  and  ha^dng  its  attachments  immediately  within  the 
umbones : the  extremities  have  a semich-cular  form,  arched  behind  and  slightly  bifid  in 
front,  indicating  its  double  nature.  This  muscle  is  perhaps  the  equivalent  of  the  diva- 
ricators  in  Waldlieimia,  though  it  seems  more  likely  that  it  homologically  represents 
only  the  accessory  divaricators,  which  in  position  exactly  agree  with  it.  And  that  por- 
tion of  the  intestine  which  corresponds  to  the  intestine  in  the  Terehratulidce,  terminates 
in  front  of  it,  at  the  ventral  wall  of  the  body,  just  as  the  intestine  does  in  them. 
The  action  of  the  divaricator  is  to  approximate  the  umbonal  regions,  and  thus,  press- 
ing forward  the  fluid  in  the  perhisceral  chamber,  to  open  the  valves  in  front.  Other 
muscles  are,  however,  likewise  employed  to  assist  in  this  office.  The  lateral  walls  ^ of  the 
body  are  thick  and  strong,  being  amply  supplied  with  transverse  muscular  fibres  which 
pass  from  one  valve  to  the  other.  The  fibres  are  strongly  developed  towards  the  poste- 
rior extremity  of  the  chamber,  Avhere  they  form  a thick  inner  layer.  These  may  be 
denominated  the  posterior  parietal  muscles.  The  walls  of  the  anterior  recess  of  the 
' Plate  LXVI.  figs.  1,  3,  4. 
5 m-2 
