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790 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
the greater part of the Lias, occasionally occur, but most frequently in 
the calcareous zones. The chief genera are Cerithium (40 species), 
Turbo (31), Trochus (27), Tectaria (Eucyclus) (22), Plewrotomaria (23), 
and Chemnitzia (19). The cephalopods, however, are the most abundant 
and characteristic shells of the Lias; the family of the ammonites num-~ 
bers upwards of 130 species. Many of these are the same as those that 
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have been found in the Jurassic series of Germany, and they occupy on ~ 
the whole the same relative horizons, so that over central and western 
Europe it has been possible to group the Lias into the various zones 
given in the table (p. 786). The genus Nautilus is represented by nine 
or morespecies. The dibranchiate cephalopods are represented by about 
60 species of the genus Belemmnites. 
From the English Lias many species of fishes have been obtained. 
Some of these are placoids, known only by their teeth (Acrodus, Ceratodus), 
others only by their spines (Nemacanthus), and some by both teeth and 
spines (Hybodus). The ganoids are frequently found entire, the genera 
Dapedius, Pholidophorus, Aichmodus, Lepidotus, Pachycormus, and Leptolepis 
being among the most frequent. But undoubtedly the most remarkable 
paleontological feature in this group of strata is the number and variety 
of its reptilian remains. The genera Ichthyosaurus, Plesiosaurus, Dimor- 
phodon, Megalosaurus, Teleosaurus, and Steneosaurus have been recovered, 
in some cases the entire skeleton having been found with almost every 
bone still in place. | 
The Lias extends continuously across England from the mouth of the 
Tees to the coast of Dorsetshire. It likewise crosses into South Wales. 
An interesting patch occurs far removed from the main mass of the 
formation, lying unconformably on Triassic beds at Carlisle. A consider- 
able development of the Lias stretches across the island of Skye and 
adjoining tracts of the west of Scotland, where the shore-line of the 
period is partly traceable. In the north of Ireland also the characteristic 
shales appear in several places from under the Chalk escarpment. 
The Lower or Baru Oouires lie conformably upon the top of the 
Lias, with which they are connected by a general similarity of organic 
remains, and by about 40 species which pass up into them from the 
Upper Lias. ‘They consist in the south-west and centre of Hngland of 
shelly marine limestones, with clays and sandstones, but, traced north- 
wards into Northampton, Rutland, and Lincolnshire, they pass into a 
series of strata indicative of deposit in the estuary of some river 
descending from the north, for, instead of the abundant cephalopods of 
the truly marine and typical series, we meet with fresh-water genera 
such as Cyrena and Unio, marine forms such as Ostrea and Modiola, thin 
seams of lignite, thick and valuable deposits of ironstone, and remains of 
terrestrial plants. ‘These indications of the proximity of land become. 
still more marked in Yorkshire, where the strata (800 feet thick) consist 
chiefly of sandstones, shales with seams of ironstone and coal, and occa- 
sional horizons containing marine shells. It is deserving of notice that 
the Cornbrash, which forms the top of the Lower Oolite in the typical 
Gloucestershire district, occurs likewise in the same position in York- 
shire. ‘Though rarely more than 8 feet thick, it runs across the country 
from Devonshire to Yorkshire. ‘Thus a distinctly defined series of beds 
of an estuarine character is in the north homotaxially representative 
of the marine formations of the south-west. At the close of the 
