INTRODUCTION. 3 



When we turn to our own country the same phenomena of distribution are 

 very clearly manifest, though on a small scale. The Lower Jurassic maximum is 

 shown by the Lower Lias extending from Lyme Regis to Yorkshire and beyond to 

 Dunrobin, Arran, and Antrim ; and it is followed by the changing and decreasing 

 strata of marine formations from the Oolites to the Forest Marble in the south, the 

 Estuarine Clays and Northampton Sands in the centre, and the plant beds extending 

 to the Coal of Brora in the north. Again, the Upper Jurassic maximum com- 

 mences with the Cornbrash, whose constancy is only equalled by that of the 

 succeeding Oxfordian strata, whose basal bed it forms, and extends with scarcely 

 a noticeable change from Weymouth to Scarborough, and is represented at a 

 slightly later date by the " roof-bed " of the Brora Coal. 



Such a change as this from constancy to variability would seem to indicate 

 a true basis of classification into two parts — the Upper and the Lower, each 

 representing a distinct cycle of deposits. In the presence of this coincidence of 

 the development of certain types of Ammonites, with an almost world-wide 

 expansion of the rocks containing them, the accidents of their local form or colour 

 sink into insignificance. Such accidents seemed quite natural in the days of 

 Conybeare and Quenstedt, though the line of division between the Lower and 

 Middle Oolites of the former in no wise coincided with that between the Brown 

 and White Jura of the latter. It was only when further travel introduced the 

 knowledge of the wider distribution of the Upper Jurassic that the significance 

 of the zone of Macrocephalites macrocephalus as the base of a new series of 

 deposits was perceived, and the meaning of the constancy of the Cornbrash, as 

 of that of the Rhastic, was indicated. 



Yet it is quite true, as indicated by Phillips, that in the South- West of England, 

 where both series are equally developed, the fossils of the Cornbrash commonly 

 observed are " not much different in general aspect from that of the Great Oolite 

 below them." l This is because only the so-called " demoid " fossils are commonly 

 observed, but when all the fauna is examined it may happen, and it does, that 

 there are a crowd of new forms of a higher type, showing alliances with the strata 

 above rather than with those below. To determine these forms is one of the 

 objects of the present monograph. 



From the above considerations it appears that the critical line to draw is the 

 basal line. Often the change of character in the strata below the Cornbrash is 

 characteristic — as clay in the central counties, or as estuarine sands in Yorkshire. 

 In these cases there is no difficulty in recognising this line, but when limestone 

 follows limestone, as in the south-western portion of its course, where the highest 

 bed of the Lower Oolites is the Forest Marble, more care is required to distinguish 

 between the two. Consequently I have accepted as belonging to the Cornbrash 



1 ' Geology of Oxford,' 1871, p. 155. 



