94 JURASSIC CHRONOLOGY" : LIAS. [vol. lxxvi. 



Wessexian 1. Phricodoceras. 

 Raagayan 7. leckenbyi. 

 Deiran 8. lymense. 



7. Radstoclcicerax. 



6. Gleviceras. 



4. oxijnotum. 



Of these Gloucestershire shows only preserved strata of the two 

 lowest, of which the Gleviceras deposits seem from this investi- 

 gation to fail in places. The other episodes of bigness appear to 

 fail altogether. 



The Oxynoticeras-oxynotum fauna would not be considered 

 large, because, as already noted, the specimens usually found are 

 truncated — so much so as to be colomorphs sometimes ; but it must 

 be reckoned by the living size, and not by the fossil size. 



The deductions to be drawn, therefore, seem to be that the 

 strata of Hierlatz and Spezia were not deposited in a landlocked 

 sea unfavourable to ammonites ; that no special pak-eogeographical 

 delineation is recmired to meet their case ; that micromorph 

 ammonites, like those of the Sclilotheimia-lacunata series, had a 

 wide European extension — Spezia, Hierlatz, Wiirtemberg, Rhone 

 Basin, Gloucestershire. Yorkshire ; that the time when they lived 

 was a time of small ammonites ; that in this case, and in others, 

 the deposits of the dates when small ammonites lived happen to 

 have been preserved at Hierlatz and Spezia, while those when large 

 ammonites flourished happen to have been removed : that much 

 the same phenomena are found in Gloucestershire extending over 

 a longer range of time ; but that the opposite phenomena — the pre- 

 servation of strata laid down while large ammonites nourished, and 

 in some cases the loss of strata deposited while small ammonites 

 lived — are found in the contiguous area of North Somerset. There- 

 fore the small size of the ammonite fauna of Hierlatz and Spezia, 

 in so far as they are micromorphs, is a chronological, not a geo- 

 graphical, phenomenon due to diastrophic movements accidentally 

 preserving strata of certain dates ; and, in so far as they are 

 colomorphs, the same diastrophic agencies may be the cause — the 

 colomorphs being small, because reduced by geological agency in 

 the course of earth-movements causing recleposition of strata and a 

 breaking-off of the outer whorls of ammonites. 



When these studies of Jurassic Chronology are more advanced, 

 it may be possible to present a complete plotted table showing 

 chronologically the rise and fall in size (diameter) of British 

 Jurassic ammonites, which should be of considerable interest ; but 

 at present collection -failure and publication -failure in regard to 

 large forms might vitiate the results. In any case, the collection 

 of data is no small task. For instance, only lately have I obtained 

 evidence of a megalomorph in the Upper Kimmeridge Clay — post- 

 viryula — in the shape of a fragment of commencement of body- 

 chamber of a Perisjjhinctes of the rotundus type as interpreted 



