Vol. 54.] DIYISIONS OF SO-CALLED ' JURASSIC ' TIME. 443 



during whicli the strata of the Jurassic System were deposited. On 

 the other hand Mesozoic is a chronological, and not applicable 

 as a strati graphical, term. 



This note is inserted for the purpose of explaining the reason for 

 the changes introduced since the paper was read before the Society. 

 They are only changes of name ; the principle of the paper remains 

 the same. — June, 1898.] 



I. General Considerations. 



In the present communication I have two main purposes in view. 

 The first is to demonstrate the application of the chronological 

 system of nomenclature in regard to a certain portion of time ; the 

 second is to construct what may be called a geological calendar, 

 in order that those who are collecting fossils may be able to date the 

 species which they find. Particularly do I give this for the benefit 

 of those who are concerned with the biological aspect of palaeontology, 

 to whom a knowledge of the exact succession of organic beings in 

 time is an important feature, to enable them to study the evolution. 



It may be noted, with one or two exceptions, to be dealt with 

 later, that all the hemeral names are taken from the names of 

 ammonites. For the divisions of Mesozoic time this plan would be 

 adopted so far as possible ; and it needs no defence, for Hang 

 remarks with regard to the zonal work of Oppel, Quenstedt, 

 Neumayr, and others: — ' L'importance des ammonites pour I'eta- 

 blissement du parallelisme des couches etait reconnue, et en meme 

 temps se trouvait pose le principe que les autres fossiles marins, 

 tels que gastropodes, lamellibranches, brachiopodes, echinodermes, 

 ne devaient intervenir qu'en seconde ligne dans les assimilations. 

 Quant aux caracteres lithologiques, ils etaient naturellement 

 refoules tout-a-fait a Tarriere-plan.'^ 



Thus the next step in the arrangement follows by logical 

 necessity upon this one; for if the ammonites are recognized as the 

 best indicators of the faunal sequence, and since the chronological 

 subdivisions depend upon this sequence, then the further grouping of 

 the chronological subdivisions must be controlled by the zoological 

 afiinities of the ammonites. Eor instance, the shortest geological 

 time-division is a hemera: that is, the time during which a par- 

 ticular species — generallj^ in Mesozoic chronology, of an ammonite — 

 had dominant existence. A longer space of time contains so many 

 hemerse ; it is at present designated by the very faulty title of ' an 

 Age ' ; but it is obvious that, as the hemera depends on the 

 ammonite-species, an ' Age ' must depend on the duration of 

 allied series of ammonite-species. In the present state of ammo- 

 nite-nomenclature the duration of an Age is dependent upon the 

 duration of an ammonite-family, or at any rate, if not the whole 

 period of duration, at least that portion of time during which the 

 family was of the greatest importance as a constituent of the 

 ammonite-fauna. In other words, as a family has its periods of 



^ Article ' Jurassique ' in ' La G-rande Encyclopedic,' Paris, 1894, p. 325. 



