■2o2 
DR.  IVHEELTON  HIND  ON  THE  AFFINITIES  OF 
i  May  1893, 
Interior. — Attachment  of  mantle  marked  by  a  dotted  line  in  casts, 
corresponding  to  small  pits  in  the  shell.  Anterior  muscle-scars 
three  in  number,  pit-like.  The  anterior  is  the  largest,  and  is  situated 
in  the  umbo  ;  the  next  is  situated  just  below  the  horizontal  part  of 
the  ridge,  and  the  most  posterior  on  the  ridge,  exactly  at  the  point 
where  it  changes  its  direction  downwards.  The  posterior  scar  is 
large,  obovate  antero-posteriorly,  situated  close  to  the  posterior 
superior  angle  of  the  shell.  Faint  traces  of  byssal  muscles  are 
seen  near  the  upper  border.  Two  or  three  slight  diverging  grooves 
arise  from  the  upper  part  of  the  keel,  and  are  seen  as  ridges  in 
casts. 
Surface  ornamented  with  fine  striae  and  lines  of  growth,  which 
arise  from  the  anterior  side  of  the  umbo,  and  pass  along  the  anterior 
part,  until  they  reach  the  ridge,  wTien  they  become  separated  by 
regular  and  wider  intervals,  and  reflected  strongly,  passing  across 
the  shell  to  the  superior  border.  Periostracum  wrinkled.  Shell  mode¬ 
rately  thick. 
Dimensions: — Greatest  antero-posterior  length,  31  mm.  (1|  inch) ; 
greatest  dorso-ventral  depth,  24  mm.  (about  1  inch) ;  thickness, 
8  mm.  (-JSr  inch). 
Remarks. — This  form  varies  very  much  in  the  length  of  the 
hinge-line,  which  may  even  exceed  the  length  of  the  shell ;  also  in 
the  shape  of  the  posterior  end.  In  many  the  hinge-line  passes  by  a 
gentle  curve  into  the  posterior  border ;  in  others  the  two  borders 
join  at  an  obtuse  angle,  in  which  case  the  posterior  border  is  slightly 
sinuated  at  the  upper  part.  This  shell  is  more  nearly  triangular 
than  an}’  of  the  other  groups. 
Salter’s  figures  in  4  Iron  Ores  of  South  Wales,"  pi.  ii.  fig.  14,  and 
6  Country  round  Wigan,’  2nd  ed.  p.  37,  fig.  2,  no.  3,  and  p.  38,  fig.  3, 
have  the  long  hinge-line  and  cmarginate  posterior  end. 
In  compressed  forms  from  shales  the  periostracum  is  extremely 
wrinkled.  The  best-preserved  specimens  are  from  ironstone-beds. 
Localities. — North  Staffordshire  Coalfield:  roof  of  Hard  Mine, 
Banbury,  and  Holly  Lane  Coals,  Moss  Shale,  Knowles  Ironstone  ; 
South  Wales  :  Darran  Pins  ;  Coalbrookdale  :  White  Flats  ;  Lan¬ 
cashire  Coalfield:  llochdale,  30  feet  below  the  Arley  Mine  ;  York¬ 
shire  Coalfield  :  Wakefield. 
I  am  unable  to  identify  Brown’s  Avicula  tenua  (sic),  to  which 
Salter  gave  the  name  Anthracojotera  Browniana ,  as  belonging  to  this 
genus.  According  to  Brown’s  figures  (Trans.  Geol.  Soc.  Manch. 
vol.  i.  pi.  v.  fig.  23,  and  4  Foss.  Conch.’  pi.  lxviii.  fig.  9)  the  shell  has  a 
definite  Avicula- like  form.  Salter, 4  Country  round  A\  igan,  figs.  2,  3, 
figures  what  he  supposed  to  be  fragments  of  Brown’s  shell,  dotting  in 
an  outline  as  if  it  had  the  shape  of  an  Anthracoptera.  Both  Brown’s 
figures  appear  to  be  those  of  complete  shells,  and  probably  represent 
a  species  of  shell  hitherto  called  Posidonia  from  the  Lancashire  coal¬ 
field.  I  have  therefore  dropped  both  names  as  synonyms  of  the 
shell  described  above. 
Brown’s  figure  of  Avicula  modiolaris,  purporting  to  be  a  copy  of 
8owerby’s  in  the  4  Geology  of  Coalbrookdale,’  is  unrecognizable  ;  but 
his  description,  though  meagre,  is  evidently  of  our  shell.  The  figures 
