250  DR.  WHEELTON  HIND  ON  THE  AFFINITIES  OF  [May  1 893, 
Myalina,  McChesney,  ‘  New  Sp.  of  Palaeozoic  Fossils,’  18b0. 
,,  Meek  &  Worthen,  Proc.  Acad.  Nat.  Sci.'Pliilad.  1860. 
,,  Proc.  Acad.  Nat.  Sci.  Chicago,  1866. 
Modiola,  Brown,  ‘  Fossil  Conchology,’  1849. 
,,  Lea,  Journ.  Acad.  Nat.  Sci.  Philad.  vol.  ii.  1853. 
No  name,  R.  Garner,  ‘Nat.  Hist,  of  County  of  Stafford,’  pl.  E,  figs.  19r 
21,  22  (1844). 
Generic  Description. — Shell  modioliform,  obliquely  triangular,, 
inequivalve,  the  left  valve  being  most  tumid,  the  right  valve 
flattened,  both  valves  notched  for  the  byssus,  the  left  one  slightly 
more  so.  Hinge-line  straight.  Hinge-plate  in  some  species  almost 
obsolete,  in  others  markedly  striated.  Edentulous.  Anterior,  end 
small  and  oblique,  forming  a  lobe  anterior  to  the  umbonal  ridge. 
Beaks  almost  terminal,  slightly  pointed  anteriorly  at  apices.  Um¬ 
bonal  ridge  strong  in  left  valve,  less  marked  in  right,  extending  from 
the  umbones  to  the  posterior  part  of  the  shell,  where  it  becomes  lost 
in  most  species  before  it  reaches  the  posterior  border.  Surface 
ornamented  with  flat  concentric  lamellae,  and  lines  of  growth. and 
wrinkled  periostracnm  over  the  posterior  end.  Below  the  oblique 
ridge  the  striae  are  close,  and  arising  from  the  umbo  diverge  until 
they  reach  the  ridge,  where  they  turn  upwards  at  wider  and 
regular  intervals,  and  are  continued  to  the  superior  border. 
Interior. — The  pallial  line  is  represented  by  a  dotted  line.  The 
post-adductor  scar  is  circular,  large,  situated  near  the  posterior  end, 
4  distance  from  the  superior  to  the  inferior  border.  Anterior- 
adductor  scar  small,  pit-like,  anterior  to  the  umbo.  There  are  two 
accessory,  pit-like  scars,  for  the  pedal  and  byssal  muscles  -one 
umbonal,  and  one  midway  between  the  umbo  and  the  anterior- 
adductor  scar.  Scars  for  the  pedal  and  byssal  muscles  about  the 
centre  of  the  superior  border,  and  just  below  the  hinge-line. 
Observations. — I  have  been  fortunate  enough  to  obtain  some 
weathered-out  interiors  of  this  genus  from  the  Ten-Feet  and  Hard 
Mine  seams  of  the  North  Staffordshire  Coalfield,  which  possess  a 
well-marked,  striated  hinge-plate,  showing  the  close  affinity  of 
these  shells  with  Myalina,  under  which  they  were  once  described.. 
They  differ,  however,  in  not  possessing  triangular  septa  within 
the  beaks  for  the  insertion  of  the  anterior-adductor  muscle.  It  -is 
in  these  beds  that  the  genus  appears  in  such  profusion,  both  in 
numbers  and  in  its  specific  forms. 
Continental  authors  have  described  these  shells  as  Dreissenia ,  but 
they  are  undoubtedly  in  error.  The  absence  of  the  shelf  for  the 
anterior  adductor,  and  the  shape  and  position  of  the  posterior-muscle 
scar,  which  in  Dreissenia  is  riband-like  and  flat,  and  is  inserted 
into  the  shell  close  to  the  upper  border,  show  a  divergence  from 
Dreissenia. 
McCoy,  ‘  Brit.  Palaeozoic  Eoss./p.  492,  thinks  that  there  were  two 
anterior  adductors,  each  arising  from  the  large  anterior  muscle-pit, 
but  crossing  each  other  to  be  inserted  into  the  smaller  scar  near  the 
umbo.  This  may  be  so ;  but  from  dissections  of  Mytilus ,  Modiola r 
and  Dreissenia,  I  believe  the  uppermost  scar  to  be  that  of  the  anterior 
pedal  muscle.  The  shells  now  under  consideration  closely  resemble 
