[    301  ] 
XXXII.  On  the  Genus  Chama,  Brug.,  tvith  Descriptions  of  some  Species  apparently  not 
hitherto  characterized.  By  W.  J.  Broderip,  Esq.,  Vice-President  of  the  Geological 
and  Zoological  Societies,  F.R.S.,  L.S.,  ^c. 
Communicated  December  23,  1834. 
The  genus  Chama,  modified  as  it  was  by  Bruguieres,  includes  only  that  section  of  the 
Linnean  genus  of  the  same  name,  the  animal  of  which,  under  the  title  of  Psilopus,  has 
been  described  and  figured  by  Poll.  Lamarck  and  Cuvier  have  both  adopted  this 
arrangement  of  a  group  which  is  natural,  gregarious,  and  whose  geographical  distribu- 
tion appears  to  be  confined  to  the  warmer  seas,  the  Mediterranean  being  the  locality  of 
the  lowest  temperature  where  any  of  the  species  have  been  hitherto  found.  The  shells 
are  attached  by  their  external  surface  to  submarine  bodies,  such  as  corals,  rocks,  and 
shells,  and  have  been  observed  at  depths  ranging  from  points  near  the  surface  to  seven- 
teen fathoms.  These  shells  appear  to  be  subject  to  every  change  of  shape,  and  often  of 
colour,  that  the  accidents  of  their  position  may  bring  upon  them.  Their  shape  is  usu- 
ally determined  by  the  body  to  which  they  are  fixed  ;  the  development  of  the  foliated 
lamince  which  form  their  general  characteristic  is  atfected  by  their  situation  ;  and  their 
colour  most  probably  by  the  food  and  by  their  greater  or  less  exposure  to  light.  The 
Chama  that  has  lived  in  deep  and  placid  water  will  generally  be  found  with  its  folia- 
tions in  the  highest  state  of  luxuriancy,  while  those  of  the  individual  which  has  borne 
the  buffeting  of  a  comparatively  shallow  and  turbulent  sea  will  be  poor  and  stunted. 
Lamarck,  with  much  reason,  has  placed  the  genus  Chama  properly  so  called  between 
Biceras  and  Etheria ;  but  he  has  divided  the  species  into  two  sections,  viz.  first,  those 
the  umbones  of  whose  shells  turn  from  left  to  right,  and,  secondly,  those  whose  umbones 
turn  from  right  to  left.  M.  Sander  Rang,  in  his  Manual,  has  adopted  this  division,  to 
which  I  cannot  subscribe,  because  it  will  not  bear  the  test  of  examination.  Two  remark- 
able instances  are  now  well  known  of  regular  Bivalves  of  the  same  species,  in  which  one 
specimen  may  be  regarded  as  being  the  reverse  of  the  other,  viz.  Lucina  Childreni  and 
an  inequivalve  M/^iks  in  the  British  Museum  ;  and,  to  come  at  once  to  the  case  before 
us,  the  same  species  of  Chama  is  sometimes  attached  by  the  right,  sometimes  by  the 
left  valve ;  or,  in  other  words,  in  one  individual  of  the  species  the  umbones  will  turn 
from  left  to  right,  while  in  another  individual  they  will  turn  from  right  to  left. 
The  fossil  species  are  numerous,  and  occur  in  the  supracretaceous  group,  particularly 
in  the  Subapennine  beds,  and  those  of  Bordeaux  and  Dax  ;  in  the  cretaceous  group  ; 
and  also  in  that  of  the  oolite. 
To  me  the  distinction  of  the  species  of  this  genus  appears  to  be  difficult.  Their 
VOL.  I.  PART  IV.  2  s 
