482 
ME.  S.  S.  ETJCEMAX  OX  THE  BAJOCIAX 
[Nov.  1893? 
should  mark  the  smallest  consecutive  divisions  which  the  sequence 
of  different  species  enables  us  to  separate  in  the  maximum  develop¬ 
ments  of  strata.  In  attenuated  strata  the  deposits  belonging  to 
successive  hemerae  may  not  be  absolutely  distinguishable,  yet  the 
presence  of  successive  hemerae  may  be  recognized  by  their  index- 
species,  or  some  known  contemporary ;  and  reference  to  the  maximum 
developments  of  strata  will  explain  that  the  hemerae  were  not  con¬ 
temporaneous  but  consecutive. 
The  term  ‘  hemera  ’  is  intended  to  mark  the  acme  of  de¬ 
velopment  of  one  or  more  species.  It  is  designed  as  a  chrono¬ 
logical  division,  and  will  not  therefore  replace  the  term  4  zone J  or 
be  a  subdivision  of  it,  for  that  term  is  strictly  a  stratigraphical  one. 
Our  present  4  zones  ’  give  the  false  impression  that  all  the  species 
of  a  zone  are  necessarily  contemporaneous  ;  but  the  work  of 
Munier-Chalmas  in  Normandy,  and  my  own  labours  in  other  fields, 
show  that  this  is  an  incorrect  assumption.  The  term  4  hemera  will 
therefore  enable  us  to  record  our  facts  correctly ;  and  its  chief  use 
will  be  in  what  I  may  call  4  palaeo-biology .’ 
Description  of  the  Strata. — Anyone  unacquainted  with  the 
Dorset-Somerset  Inferior  Oolite,  and  the  richness  of  its .  deposits, 
would  scarcely  credit  the  difficulty  experienced  in  these  investiga¬ 
tions  from  want  of  names  for  the  Ammonites.  Species  which  have 
been  perfectly  well  known  for  years  as  indicators  of  certain  horizons 
are  altogether  devoid  of  any  specific  name.  They  could  not, 
therefore,  be  recorded  with  precision — only  the  generic  name  could 
be  given. 
In  this  connexion  the  great  value  of  the  new  generic  names 
becomes  most  apparent.  By  their  aid  it  is  possible  to  narrow  the 
identity  of  new  or  unnamed  species  within  reasonable  limits,  and 
to  record  the  fact  with  brevity  in  a  paper.  With  their  help, 
the  characteristics  and  chief  features  of  unnamed  species  can  be 
described  in  a  way  comprehensible  to  other  workers  in  the  same 
field,  by  the  use  of  a  single  word.  Lastly,  by  employing  them  in 
the  case  of  known  species,  the  remarkable  changes  in  the  A.mmonite- 
faunae  of  successive  deposits  can  be  illustrated ;  and  this  important 
fact  it  is  impossible  to  show  by  anjr  other  means. 
I  consider  the  strata  dealt  with  in  this  paper  to  have  been 
deposited  during  at  least  twelve  hemerae  ;  and  the  names  of  these 
hemerae  will  appear  in  the  Sections  at  the  tops  of  the  respective 
divisions  of  strata  assigned  to  them.  These  hemerae1  are  as  follows  : 
1.  Fuscce  and  2.  Zigzag,  v  ( Oppelia  fusca ,  Quenst.,  sp.,  and 
4  Stephanoceras ’  zigzag ,  d’Orb.,  sp.)  Owing  to  the  unfossili- 
ferous  nature  of  the  strata  deposited  during  these  hemerae,  in 
this  district,  an  exact  apportionment  of  the  beds  to  each  one 
1  Xu  connexion  with  the  specific  names  which  designate  the  hemera;,  I  have 
considered  it  advisable  to  use  the  genitive  inflexion  as  more  truly  expressing 
what  is  intended.  Thus  ‘  fuscce  hemera,’  which  is  short  for  ‘  Oppdi<B  fusc<s 
hemera  ’  =  the  hemera  of  the  tawny  Oppelia,  means  the  hemera  of  the  tawny 
one  {Oppelia  understood) ;  whereas  ‘fused  hemera  ’  might  seem  to  suggest  the 
‘  tawny  hemera.’ 
