444 MR. S. S. BFCKMAN ON THE GEOUPING OF SOME [Aug. 1 898, 



rise, of maturity, and of decline (or, in scientific language, its 

 epacme, acme, and paracme), so the duration of the Age would be 

 principally governed by the period of acme, and would be less con- 

 cerned with the epacmastic and paracmastic periods of the same 

 family ; because during any one such period should be found the 

 acme of another family. 



Following on this again as a logical conclusion is the recognition 

 that any time-division larger than an Age must depend upon the 

 duration in time of allied ammonite-families. When it is found 

 that a certain number of ammonite-families allied from a zoological 

 and evolutionary point of view flourish and disappear, to be 

 succeeded in a similar manner by another quite distinct group of 

 ammonite-families similarly allied among themselves, then it is 

 obvious that, so far as is practicable, the time- division which 

 embraces so many Ages should correspond to the period of time 

 marked by this zoological phenomenon, and that the end of this time- 

 division and the beginning of the next should coincide, as far as 

 practicable, with the time when the one family-group has declined 

 so low as to become unimportant, while a succeeding family-group 

 has risen to importance. 



Such are the theoretical considerations which should guide the 

 application of a chronological scale. Their practical application, 

 however, may not fully conform to the theoretical premisses 

 because, in the first place, our knowledge of the faunas is very 

 incomplete, even in connexion with the most fully-explored regions ; 

 and secondly, the faunal successions (as known to us) do not follow 

 with that faultless regularity which would be necessary for the 

 complete fulfilment of the conditions laid down. All that is possible, 

 therefore, is to so arrange the chronological scale that while it 

 expresses what we do know, it shall be able to show good reason 

 for the arrangement where the actual facts of faunal succession do 

 not possess the regularity which would be desired to make the 

 arrangement perfect and indisputable. 



The first point for consideration, therefore, is the date at which 

 that division of the Mesozoic Era to be known as the 'Jurassic' 

 Period should commence. Hitherto the stratigraphical division 

 between Triassic and Jurassic deposits has been generally made at 

 the base of thepZanor6is-zone. The change of lithic conditions and 

 the incoming of ammonites in many places seem to make this 

 horizon peculiarly distinct ; but in a chronological scale these 

 matters have not the least weight: the only thing to be taken into 

 consideration is the zoological relationship of the ammonite-fauna 

 of the \)T:e-rotiformis hemerae in comparison with the fauna of its 

 successors and predecessors. Eirst there are three genera in the 

 ■pre-rotiformis hemerae which are closely allied to one another — 

 Psiloceras, WcpJmeroceras, and Schlotheimia. If these genera were 

 the parents of the forms which dominate the rotiformis and suc- 

 ceeding hemerae, then it would be correct to place the ipTe-7'otiformis 

 hemerae with their successors ; but in my opinion they are not the 



