ME.  HAJS^COCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
799 
the  two,  when  the  muscle  is  exposed  at  the  surface  of  the  body ; thus  seen,  the  extre- 
mity has  somewhat  the  shape  of  a battledoor;  and  the  pair  meeting  behind  almost 
enthely  enclose  the  occlusors  and  the  terminal  extremity  of  the  intestine.  Though  thus 
altered  and  combined  into  one,  the  function  of  this  single  pair  of  muscles  will  be  exactly 
the  same  as  when  divided  into  two,  as  the  areas  of  attachment  are  not  materially  changed. 
It  will,  in  fact,  open  the  valves  and  at  the  same  time  protrude  the  peduncle ; but  on 
account  of  the  combination,  the  two  actions  may  perhaps  be  rendered  more  harmo- 
nious, and  probably  somewhat  invigorated. 
Another  and  more  important  modification  obtains  in  W.  cranium  and  T.  ca'put-ser- 
pentis\  In  these  two  species  the  dorsal  adjustor  muscles  are  not  attached  to  a hinge- 
plate,  as  in  W.  Australis,  but  have  them  msertions  in  the  valve  itself,  and  are  very  large 
and  poweiiiil.  In  both  species  the  superior  extremities  of  these  muscles  are  seen  at  the 
sui'face  of  the  animal,  on  each  side  of  the  median  line,  elongated  in  the  antero-posterior 
du’ection,  and  extending  between  the  occlusors  almost  as  far  forward  as  theh  anterior 
margins.  They  are  enlarged  posteriorly,  particularly  in  T.  ca])ut-serpentis,  in  which  they 
are  of  an  u’regular  form. 
The  divaricators  and  the  accessory  divaricators  in  W.  cranium‘s  are  likewise  united  in 
the  same  manner  as  in  the  species  before  alluded  to. 
The  modification  of  the  dorsal  adjustor  muscles  cannot  be  of  much  importance  in  a 
physiological  point  of  view ; it  will,  however,  increase  their  power  in  proportion  as  the 
area  of  their  insertions  is  extended.  Moreover,  these  muscles  are  largely  developed  as 
well  as  the  ventral  pah.  It  would  therefore  seem  probable  that  the  movements  of  the 
shell  upon  the  peduncle,  in  these  species,  may  be  more  extensive  and  livelier  than  usual. 
But  to  the  palaeontologist,  to  whom  the  muscular  impressions  in  fossils  are  of  great 
value,  these  modifications  cannot  fail  to  possess  considerable  interest,  evincing  as  they 
do  to  what  extent  the  arrangement  of  the  muscles  may  be  modified  without  any  essen- 
tial change  in  the  economy  of  the  animal.  In  the  dorsal  valve  there  may  be,  as  in  the 
above  examples,  as  many  as  six  muscular  impressions  instead  of  four,  and  in  the  ventral 
there  may  be  two  less  than  usual ; facts  sufficiently  puzzling  without  the  light  of  anato- 
mical investigation.  It  is  also  worthy  of  remark,  that  in  those  species  with  the  dorsal 
adjustor  muscles  attached  to  the  valve  there  is  no  hinge-plate®;  and  therefore  it  may  be 
presumed  that  in  the  fossil  species  with  this  plate  deficient  the  muscles  will  be  arranged 
in  the  same  manner. 
In  Bhynchonella  j[)sittacea‘^  the  general  disposition  of  the  muscles  is  the  same  as  in 
W.  australis,  only  they  are  longer  and  more  slender,  particularly  the  occlusors,  which, 
with  the  exception  of  their  extremities,  are  thin  and  tendinous.  The  surfaces  of  attach- 
ment also  vary  a little  in  form,  as  can  be  readily  seen  on  removing  the  shell.  The  extre- 
mities of  the  muscles  are  then  observed,  as  usual,  at  the  ventral  surface  of  the  animal, 
clustered  together,  a little  in  advance  of  the  umbonal  region.  The  occlusors  are  almost 
* Plates  LIII.  figs.  1,  3,  5 ; LXII.  fig.  2.  ^ Plate  LIII.  fig.  4. 
^ Plate  LII.  fig.  5.  ^ Plates  LX.  figs.  1,  2 ; LXI.  figs.  1,  2. 
AIDCCCLVIII.  5 M 
