Yol.  49.] 
AXTURACOPIERA  AXD  AXTHRACOWYA. 
269 
rounded,  with  a  blunt  swelling  which  passes  from  the  umbones  to 
the  posterior-inferior  angle  of  the  shell ;  the  surface  above  this 
swelling  is  compressed  into  the  hinge-line.  There  is  a  very  slight, 
broad,  oblique  sinus  anterior  to  the  ridge.  The  hinge-line  and 
ventral  margin  are  nearly  parallel,  the  latter  being  sinuate  in  its 
middle  third.  The  umbones  are  prominent,  raised  above  the  hinge¬ 
line,  and  situated  at  ^  length  of  the  latter,  which  is  not  quite  so 
long  as  the  shell,  from  the  anterior  end.  The  beaks  are  apart. 
Lunule  large  and  elongated.  Cartilage  external. 
Interior  unknown. 
Exterior. —  Surface  almost  smooth,  with  nearly  obsolete  striae  and 
lines  of  growth.  Periostracum  strongly  wrinkled. 
Size.—  Antero-posterior  measurement,  20  mm.  (i  inch)  ;  dorso- 
ventral  (at  umbo),  7  mm.  (y5^  inch) ;  from  side  to  side,  5  mm. 
(i  inch). 
Remarks. — Salter  observes  that  the  more  convex  form  and  central 
beaks  distinguish  this  species  from  A.pumila,  to  which  it  most 
nearty  approaches.  This  appears  to  be  a  very  rare  form. 
Localities. — South  Wales  :  Xo.  6  pit,  Victoria,  over  the  Three- 
quarter  Coal ;  Hard  Wine,  Longton,  Xorth  Staffordshire. 
Axtheacoaita  ptbiila.  (PI.  X.  figs.  17,  18,  18  a,  19,  28,  and  29.) 
Salter,  Geol.  Surv.  Mem.  ‘Iron  Ores  of  South  Wales,’  pl.  ii.  fig.  10,  p.  230. 
Specific  Characters. — Shell  transversely  oblong,  slightly  convex. 
Hinge-line  and  ventral  margin  nearly  straight,  almost  parallel. 
Umbones  small,  apices  depressed,  situated  about  3  length  of  the 
hinge-line  from  the  anterior  end.  Lunule  narrow  and  elongated. 
Anterior  end  short  and  somewhat  pointed,  with  or  without  a  slight 
oblique  depression.  The  posterior  end,  which  is  bluntly  rounded 
and  truncate,  has  an  obtuse  diagonal  ridge  arising  at  the  umbones 
and  passing  to  the  posterior-inferior  angle  of  the  shell.  Gently 
flattened  and  slightly  expanded  above  the  ridge. 
Interior. — Anterior  and  posterior  muscle-scars  as  in  other  species 
of  this  genus.  Pallial  line  simple,  ligament  external. 
Exterior. — Surface  covered  with  flue  strise  and  lines  of  growth. 
Periostracum  wrinkled. 
Size. — Antero-posterior  measurement,  19  mm.  (about  -i  inch) ; 
dorso-ventral,  10  mm.  (|inch);  from  side  to  side,  7  mm.  (y^inch). 
Remarks. — I  feel  very  uncertain  as  to  this  form,  as  I  have  been 
unable  to  see  the  type-specimen.  I  think  it  very  probable  that  the 
form  here  described  may  eventually  be  recognized  as  the  young  of 
the  commonest  species  in  the  Hard  Mine  Beds  of  Xorth  Staffordshire, 
to  which  I  have  given  the  name  of  A.  elongata  (see  p.  266).  If 
this  prove  to  be  the  case,  the  name  of  pumila  will  have  to  be 
removed.  Salter’s  figure  shows  the  posterior  end  to  be  less  in  the 
dorso-ventral  dimensions  than  is  the  case  with  the  family  generally. 
