MAGNESIAN LIMESTONE AND MARL SLATE. ' 387 
Middle Permian — Magnesian Limestone and Marl-slate.— 
This formation is seen upon the coast of Durham and York¬ 
shire, between the Wear and the Tees. Among its charac- 
Fig. 410. 
Schizodiis Schlothehni, Geinitz. 
Permian crystalline lime¬ 
stone. 
Fig. 411. 
The hinge of Schizodus 
truncatu,% King. Per¬ 
mian. 
Fig. 412. 
Mytilus septifer, King. 
Syn. Modiola acumi¬ 
nata^ Sow. Permian 
crystalline limestone. 
teristic fossils are Schizodus Schlotheimi (Fig. 410) and My¬ 
tilus septifer (Fig. 412). These shells occur at Hartlepool 
and Sunderland, where the rock assumes an oolitic and bo- 
tryoidal character. Some of the beds in this division are 
ripple - marked. In some parts of the coast of Durham, 
where the rock is not crystalline, it contains as much as 44 
per cent, of carbonate of magnesia, mixed with carbonate 
of lime. In other places — for it is extremely variable in 
structure—it consists chiefly of carbonate of lime, and has 
concreted into globular and hemispherical masses,* varying 
from the size of a marble to that of a cannon-ball, and radi¬ 
ating from the centre. Occasionally earthy and pulverulent 
beds pass into compact limestone or hard granular dolomite. 
Sometimes the limestone appears in a brecciated form, the 
fragments which are united together not consisting of for¬ 
eign rocks but seemingly composed of the breaking-up of 
the Permian limestone itself, about the time of its consoli¬ 
dation. Some of the angular masses in Tynemouth cliflT are 
two feet in diameter. 
The magnesian limestone sometimes becomes very fossilif- 
erous and includes in it delicate bryozoa, one of which,. 
stella retiformis (Fig. 413), is a very variable species, and has 
received many different names. It sometimes attains a large 
size, single specimens measuring eight inches in width. The 
same bryozoan, with several other British species, is also 
found abundantly in the Permian of Germany. 
The total known fauna of the Permian series of Great Brit¬ 
ain at present numbers 147 species, of which 77, or more than 
half, are mollusca. Not one of these is common to rocks 
newer than the Palaeozoic, and the brachiopods are the only 
group which have furnished species common to the more an¬ 
cient or Carboniferous rocks. Of these lAngula Crednerii 
