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The Middle Lias, or marl-stone series, has been divided into three zones; and the Lower Lias, according to the labours of Quenstedt, Oppel, Strickland, Wright, and others, into seven zones, each marked by its own group of fossils. This Lower Lias averages from 600 to 900 feet in thickness.
The whole series has been divided by zones characterised by particular Ammonites; for while other families of shells pass from one division to another in numbers varying from about 20 to 50 per cent, these cephalopods are almost always limited to single zones, as Quenstedt and Oppel have shown for Germany, and Dr. Wright and others for England.
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