168  the  geology  oe  the  district  [May  1893, 
The  fossils  enumerated  above  prove  the  Arenig  age  of'  these 
rocks.1 
2.  Beds  with  f  tuning-fork  J  Didymograpti. — These  strata 
have  been  noted  in  a  previous  paper,2  as  passing  beneath  the  Llan- 
deilo  Limestone  of  the  neighbourhood  of  Narberth.  In  the  district 
now  being  described  we  meet  with  them  in  many  places  around  the 
dome  of  the  Tetragraptus- beds,  and  they  must  immediately  succeed 
these  beds,  though  the  arrows  shown  on  the  Geological  Survey  map 
in  the  neighbourhood  of  Whitland  suggest  the  occurrence  of  an 
overfold  or  fault,  owing  to  which  the  black  shales  with  ‘  tuning-fork  ’ 
graptolites  dip  at  or  under  the  Tetragraptus- beds. 
Bidymograptus  Murchisoni  and  other  ‘tuning-fork’  graptolites 
have  been  chiefly  found  in  localities  which,  for  convenience,  may  be 
referred  to  three  north- and-south  lines.  The  first  of  these  runs  from 
Whitland  in  a  northerly  direction  to  Llanglydwen,  and  graptolites 
have  been  obtained  at  Len-y-coed,  |  mile  north-east  of  Whitland, 
and  at  LlangUdwen  itself.  Along  this  line  also  black  shales  occur 
south-west  of  Middleway  (south  of  Whitland)  containing  nodules 
showing  cone-in-cone  structure,  and  with  several  species  of  Bellero- 
phon.  These  shales  possibly  belong  to  the  beds  under  consideration, 
as  do  almost  certainly  some  shales  by  the  roadside  north  of  Pwll-y- 
rhwyaid,  south-west  of  Whitland  Abbey,  with  a  small  Caryocaris 
and  tails  of  the  same,  or  a  Theca. 
The  second  line  runs  from  St.  Clear’s  in  a  northerly  direction  to 
Mydrim.  ‘  Tuning-fork  ’  graptolites  have  been  collected  from  the 
shales  of  Pen-y-craig,  1  mile  west  of  St.  Clear’s,  and  from  those  of 
a  quarry  \  mile  north  of  Plas-parcau,  between  St.  Clear’s  and 
Mydrim.  They  have  also  been  discovered  in  the  following  places  in 
the  neighbourhood  of  the  third  line  near  Merthyr : — Melin-ricket, 
Penllwynbach,  and  Cwm  Treharn. 
No  attempt  has  been  made  to  separate  the  Upper  Arenig  Beds 
from  those  of  Lower  Llandeilo  age,  though  this  may  be  done  by 
further  work.  All  that  can  be  said  is  that  we  are  here  dealing 
with  beds  corresponding  to  Dr.  Hicks’s  Llanvirn  Group,  and  that 
they  are  newer  than  the  Tetragraptus- beds  of  Arenig  age,  and  older 
than  the  Llandeilo  Limestone. 
3.  Llandeilo  Limestone. — This  rock  is  well  shown  north  of 
Pant-dwfn,  near  St.  Clear’s,  where  it  is  of  the  character  usual  in 
this  Southern  Welsh  area,  and  contains  : — 
Aso.phus  tyrannus ,  Murch. 
Horaalonotus. 
Calymene  cambrensis,  Salt. 
Beyrrichia  complicata ,  Salt. 
Orthis  testudinaria,  Dalm. 
Strophomena  compressa,  var. 
llandeiloensis,  Day. 
It  doubtless  passes  beneath  the  Old  Bed  Sandstone  at  no  great 
distance  from  this  section,  on  both  the  east  and  west  sides. 
1  [Other  rocks,  probably  of  Arenig  age,  occur  at  and  near  Henllan  Amgoed, 
somewhat  farther  north.  These  are  grits  with  Orthis,  and  trilobite-shales  with 
abundant  specimens  of  Ampyx ;  the  specimens  obtained  from  these  localities 
cannot  be  found.] 
2  Marr  and  Roberts,  op.  cit. 
