Vol. 54.] DIVISIONS OF SO-CILLED ' JUEASSIC ' TIME. 451 



Oertainly such generic names as Oxijnotoceras and Amaltheus are 

 used for species of the Stepheoceratidan Epoch ; but this use is 

 founded only on external similarity, and not on a study of genetic 

 affinities.^ There seems therefore no reason to divide the ' Jurassic ' 

 Period into more than two Epochs : the earlier dominated by 

 ammonites of the Arietidae and Hildoceratidse, which are closely 

 allied to one another; and the later dominated by the far more 

 highly developed Stepheoceratidee, in company with the Oppelidae. 



III. The Divisions of a Portion of ' Jukassic ' Time 

 (with Table I). 



Facing p. 450 is a table of time-divisions in illustration of the 

 foregoing remarks. It will also serve as a definite time-table 

 whereby the dates of the different ' Jurassic ' species may be recorded 

 — a matter of great importance for the study of palaeontology. 



IV. A Genealogy of some * Jurassic ' Ammonite Genera 

 (with Table II, facing p. 451). 



In order to illustrate the remarks which have been made con- 

 <jerning the divisions of ' Jurassic ' time and their dependence on 

 ammonite-evolution, a table of ammonite-genealogy has been 

 constructed. However theoretical the ammonite-genealogy may 

 appear — and it is not claimed as more than an approximate sketch 

 of the probable lines of evolution — yet the table does present 

 certain facts, namely, the times of occurrence of certain particular 

 genera. It is as a record thereof that it is put forward. But on its 

 interpretation an opinion is expressed, and that opinion is connected 

 with the proposed method of subdividing ' Jurassic ' time. 



A comparison of this genealogical table with others, and the 

 reasons for its differences therefrom, cannot be given in the present 

 paper. 



It may be remarked that the species of ammonites yet unde- 

 scribed are very numerous, that of the described species only a 

 small portion are treated with detail sufficient for genealogical 

 purposes, and that therefore the data for genealogical work are 

 often very insufficient. And when all the discovered species have 

 been figured and described with full details, they will be only a 

 portion of the total fauna which must have been in existence. So 

 that genealogical data cannot be expected to be complete even then, 

 though they may be sufficient for the purpose. 



Attention may be called to the fact that in the same family a 

 genus of ammonites farther removed from the assumed parent 



1 Something more than a crenulate carina is required to warrant the attribu- 

 tion of the co?'<^a;^M6-series t o Amaltheus or to the Amaltheidae. Their ontogeny 

 and septal details must be considered, and the series will possibly be found to 

 belong to the Stepheoceratidas, not so very distant from Farkinsonm, etc. 



