18 LIAS OF ENGLAND AND WALES : 
bratula globata, T.fimbria, and Rhynclionclla cynocepkala. These 
beds have been variously named the " Clypeus grit," the " Globata 
bed," the " Fimbria stage/' and the " Cynocephala stage." These 
fossil-beds, although local, often occur at definite stratigraphical 
horizons. In other instances we have Saurian, Fish, and Insect 
Beds ; beds which may b expected to occur on various horizons. 
It is noteworthy that where formations attain a great thickness, 
they are less fossiliferous than where they are thin. Such is the 
case with certain beds in the Lower Lias near Radstock when 
compared with the equivalent beds elsewhere, and A'ith the Inferior 
Oolite of Dorset when compared with the thick series near Chel- 
tenham. Such variations indicate a paucity of sediment in certain 
areas. Similar results are met with in certain fossil-beds that 
occur at the base of formations, when some lapse of time no doubt 
occurred without any particular addition of sediment. The Brad- 
ford Clay fossil-bed, and the " Transition Bed " on top of the 
Middle Lias may be cited as instances. In such situations derived 
and phosphatised fossils sometimes occur ; and at other times the 
leading fossils of a formation may be found locally in fossil- beds, 
as is the case in the Inferior Oolite of Dorsetshire. 
The general succession in the life-forms as we ascend the 
geological scale is manifest, and it is convenient to separate 
successive assemblages of fossils under the name of " Zones." 
It is fount! that while many species are restricted according to 
the nature of the sea-bed, others have an extended range, occur- 
ring far and wide over the marine area within which the 
European Jurassic strata were deposited. The extent of this area 
probably varied from time to time during the Jurassic period, but 
during some portions of it there was connexion with strata now 
found in North Africa, Madagascar, India, Australia, North and 
South America, and the Polar Regions. The full consideration of 
these matters must however be postponed for the final volume. It 
may, however, be mentioned that some common European Jurassic 
species, or closely allied forms, have been found in all the areas 
above mentioned. 
The significant fact is that many specific forms occur over wide 
areas, and in the same relative order of succession, thus indicating 
the "practical synchronism" of the deposits in which they are 
entombed. Allowance must of course be made for time occupied 
iu migration, but the results of modern research tend to show that 
the accumulations formed during such intervals, are small compared 
with the deposits that were contemporaneous. 
Among the forms that extend over wide areas in Jurassic times, 
species of Ammonites are the most important, and next to them 
may be ranked the Brachiopods, some of the species of which 
appear to have freely migrated without reference to sedimentary 
conditions. Belemnites also are of importance, but the species 
are difficult to distinguish. 
Zones may thus be defined as paheontological horizons. They 
are assemblages of fossils that occur in a more or less definite 
sequence, and mark stages in the life-history of the rocks. They 
