GEOLOGICAL PALAEONTOLOGY 77 



more satisfactorily determined by consideration of its 

 entire fauna. The index-fossil may have been of such 

 a nature that it could not exist, or at least flourish, 

 under certain physiographical conditions, but it is 

 probable that some of its associates were more catholic. 

 It not infrequently happens that two or more organisms, 

 belonging to entirely different groups, reach acmaic 

 stages at about the same time. Thus in the Upper 

 Chalk, the Belemnite Actinocamax quadratus and the 

 Echinoid Offaster pilula both characterize the same 

 zone, which is named after the former type. But while 

 the distribution of the Belemnite is somewhat sporadic, 

 that of the Echinoid is more uniform, so that the 

 quadratus-zont can often be recognized without dis- 

 covery of a single specimen of the " name-fossil." 



Since recognition of hemerae dates back no further 

 than the early years of this century, while zone's were 

 proposed fifty years before, it is natural that the con- 

 ception of a zone as the material deposited during a 

 hemera is a modern refinement. Hence many " zones " 

 that still do good stratigraphical service have no real 

 chronological precision, but are of the nature of " sub- 

 periods." Like larger divisions of sedimentary rocks, 

 they are identified by the specific and proportional 

 qualities of their fossil-contents. While such zones can 

 be easily recognized in their middle portions, they tend 

 to have ill-defined boundaries ; in many cases they prove, 

 on further palaeontological study, to spread over several 

 hemerae. 



It may be anticipated that the stratigraphical 

 classification of the future will be graded in accordance 

 with the acmaic stages of species or species-groups 

 (hemerae), genera (stages), families (periods) and larger 

 groups (eras). In a rough way, this grouping is already 

 in use. The Lower Palaeozoic (or Proterozoic) era is 



