This for the most part consists of very regular alternations of
argillaceous (clayey) limestone and clay, or shale. It is of great
thickness, and hence for convenience has been divided into (a) _Upper
Lias_, (b) _Middle Lias_ or _Marl-stone_, and (c) _Lower Lias_. A large
number of fossils are to be found in it. Lyme Regis and Whitby are
perhaps the best known localities; the former, on account of the great
number of specimens obtained of the huge fish-lizard (_Ichthyosaurus_,
p. 24), and long-necked _Plesiosaurus_ (p. 25), besides numberless fish;
whilst the latter is renowned for its jet (or fossilized wood) and its
"snake-stones" (_Ammonites_), concerning which curious old stories are
told. _Ammonites_ are plentiful in the Lias, which has been subdivided
into zones, or layers, named after the ammonite occurring in greatest
numbers in that particular zone. There is one thin limestone band in the
Marlstone composed entirely of the shells of _Ammonites planicostatus_.
A curious kind of oyster (_Gryphaea incurva_), locally known as the
devil's toenail, a huge _Lima_ (_L. gigantea_), a magnificent Encrinite
(_Extracrinus Briareus_), and numerous other fossils, are also to be
obtained by patient search.
[Illustration: _Belemnitas elongatus_(from the Lias).]