ME.  HANCOCK  ON  THE  OEOANIZATION  OF  THE  BEACHIOPODA. 
841 
In  Bhynchonella'^  a further  change  is  perceptible;  there  are,  however,  as  in  the  last 
species,  four  genital  sinuses  of  much  the  same  character ; but  the  two  in  the  dorsal  lobe 
give  off  no  branches.  These  would  seem  to  correspond  to  the  outer  sinuses  in  Wald- 
heimia.  The  inner  ones  are  represented  by  two  trunks  which  come  from  the  perivisceral 
chamber,  one  on  each  side  of  the  median  line,  between  the  expanded  bases  of  the  occlusor 
muscles ; they  diverge,  and  turning  outwards  and  backwards  circle  round  a little  in 
advance  of  the  anterior  margin  of  the  genital  sinuses,  giving  off  numerous  branches. 
The  branches  divide  three  or  four  times,  and  as  the  ultimate  twigs  approach  the  marginal 
vessel  they  invariably  bifurcate.  The  sinuses  of  the  ventral  lobe  have  their  anterior 
margins  next  the  median  line  prolonged  each  into  a stout  trunk,  which  soon  divides  into 
two  portions,  one  of  which  advancing  inclines  inwards,  the  other  directed  outwards  and 
backwards  turns  round  a little  in  front  of  the  genitalia.  These  two  portions  give  off 
several  branches  to  the  pallial  margin  similar  to  those  of  the  other  lobe. 
The  peri\isceral  chamber  of  lAngula  anatina^^  though  constructed  precisely  as  in  the 
other  Brachiopods,  is  considerably  modified  in  form.  It  is  much  larger  than  usual, 
extending  forward  nearly  half  the  length  of  the  shell ; and  is  depressed  and  elongated, 
with  the  dorsal  and  ventral  walls  very  delicate,  transparent  and  membranous ; the  lateral 
ones  strong,  opake,  and  muscular.  The  latter  are  firmly  attached  by  their  margins  to  the 
valves,  defining  uith  great  distinctness  the  exact  boundaries  of  the  chamber.  The 
anterior  wall  slopes  forward  towards  the  dorsal  valve  to  accommodate  the  advanced  posi- 
tion of  the  occlusor  muscles ; thus  forming  a sort  of  anterior  pouch  or  recess,  through 
the  centre  of  which  the  oesophagus  passes. 
There  are  four  pallial  sinuses,  two  in  each  lobe,  which  open  into  the  cavity  in  front, 
one  at  each  side  between  the  attachments  of  the  posterior  occlusors  and  the  adjustor 
muscles.  These  sinuses  pass  forward,  converging  until  at  the  anterior  margin  of  the  lobe 
they  almost  meet  on  the  median  line.  Long,  however,  before  they  reach  their  termina- 
tions they  suddenly  contract,  and  lose  the  character  of  trunks.  They  give  off  from  their 
outer  margins  numerous  close-set,  simple,  parallel  branches,  which  vary  in  size,  generally 
alternating,  large  and  small,  and  pass  towards  the  circumference  in  a radiating  manner. 
On  reaching  the  thickened  border  of  the  mantle  they  abruptly  contract^,  and,  as  much- 
attenuated  twfigs,  terminate  apparently  in  blind  sacs  a little  within  the  free  margin. 
These  branches  are  much  bulged  out  into  the  pallial  chamber,  and  when  not  distended 
with  fluid  assume  the  form  of  plaits,  giving  to  the  mantle  a regular  laminated  appear- 
ance. Small  branches  also  proceed  from  the  inner  margin  of  the  main  trunks,  which 
for  the  most  part  pass  inwards  and  backwards,  and  end  in  csecal  extremities.  There  is 
also  a posterior  branch  which  springs  from  the  root  of  the  great  trunk  and  turns  imme- 
diately backwards,  runs  along  the  lateral  expansion  of  the  mantle  outside  the  adjustor 
muscles,  and  terminates  behind  the  divaricator  muscle,  giving  off,  all  the  way  from  either 
' Plate  LX.  figs.  1,  2,  3.  = Plates  LXIV.  figs,  1,  2,  4,  5 ; LXV.  figs.  1,  3. 
' Plate  LXIV.  fig.  3. 
5 R 2 
