900  DR.  WHE  ELTON  HIND  ON  THE  AEFINITIE3  OF  [May  1 893, 
for  we  may  safely  assume  that  the  forms  were  surviving  in  the 
seas  of  that  period. 
The  thick,  wrinkled  periostracum  possessed  by  these  shells  was 
one  of  the  characters  on  which  Salter  relied  as  a  generic  peculiarity. 
This  feature  would  be  considered  typical  of  a  freshwater  habitat 
among  recent  molluscs ;  it  is  indeed  necessary  in  these  forms,  to 
prevent  excessive  erosion  by  the  carbon  dioxide  dissolved  in  rivei 
and  lake  waters,  and  thus  it  constitutes  an  additional  link  in  the 
chain  of  evidence  of  the  freshwater  origin  of  the  Coal  Measures. 
The  other  affinities  of  Anthracoptera  and  Antliracomya  to  recent 
freshwater  shells  afford  strong  presumptive  evidence  of  the  fresh¬ 
water  origin  of  the  greater  part  of  the  Coal  Measures  ;  and  the  occa¬ 
sional  simultaneous  presence  of  fluviatile  forms  with  marine,  said  to 
occur  in  a  few  beds,  may  be  easily  accounted  for  by  the  supposition 
that  they  inhabited  estuarine  and  tidal  rivers,  whence  some  would 
be  washed  out  seawards,  and  so  become  mingled  with  marine  types. 
I  must  confess,  however,  that  I  have  never  seen  this  intermixture 
of  forms,  and  can  obtain  no  evidence  of  it  from  many  fellow-workers 
in  various  coalfields.  Salter,  Geol.  Surv.  Mem.,  ‘  Country  round 
Wman/  2nd  ed.  p.  34,  quotes  the  occurrence  of  Anthracosia  and 
Goniatites  not  actually  in  the  same  layers,  but  closely  intermixed, 
and  states  that  Anthracosia  acuta  is  found  in  “undoubtedly  marine 
beds  at  Clitheroe.  (I  cannot  understand  the  last  allusion.) 
Anthracohya  Adamsii.  (PI.  A  III.  figs.  5,  5  a,  6,  / ,  8.) 
Salter  Geol.  Surv.  Mem.,  ‘  Iron  Ores  of  South  Wales,’  p.  230,  pi.  ii.  fig.  7. 
Kon  Ward,  Trans.  N.  Staffs.  Inst.  Min.  &  Mech.  Eng.  vol.  x.  (1890)  p.  125, 
pi.  i.  fig.  2. 
Specific  Characters.— Shell  obliquely  ovate,  compressed,  ventral 
margin  much  curved,  narrower  at  the  short,  rounded,  anteiior  end, 
broadest  at  about  ^  distance  from  the  posterior  margin,  which  is 
obliquely  truncate.  Anterior  end  short,  moderately  convex,  straight 
above,  rounded  and  sinuated  below.  Posterior  end  flattened  and 
expanded.  Valves  slightly  unequal,  the  left  being  more  convex. 
An  oblique,  obtuse  swelling  passes  diagonally  from  the  umbo,  across 
the  valves  ;  the  byssal  furrow  is  broad  and  shallow,  but  there  is  no 
perceptible  marginal  notch  ;  lunule  elongate  and  narrow.  Hinge¬ 
line  straight,  ^  length  of  the  shell.  Edentulous  cartilage  external, 
in  grooves  on  each  side  of  the  hinge.  Embones  broad,  tumid,  raised 
above  the  hinge-line,  almost  contiguous,  situated  -S-  distance  from 
the  anteiior  end  of  the  hinge-line.  Shell  thin. 
Interior  smooth. 
Exterior  marked  with  fine  eccentric  lines  and  folds  of  giov\th, 
starting  from  the  anterior  end,  close  together,  but  separating  as  the) 
pass  across  the  shell  in  regular  curves,  to  reach  the  posterior  part  of 
the  hinge-line  and  the  posterior  border.  Periostracum  stiongly 
wrinkled. 
Size. — Antero-posterior  measurement,  65  mm.  (2J-  inches) ;  from 
side  to  side,  10  mm.  (-J  inch)  ;  dorso- ventral  measurement  at  j  from 
posterior  end  (the  greatest  depth),  45  mm.  (14  inch). 
