WENLOCK FORMATION. 
467 
542. 
Fig. 543. 
Sphcerexochus mirus, 
Beyrich; coiled np. 
WeDlock Limestone, 
Dudley; also found 
in Ohio, N. America. 
Fie. 544. 
size and flattened form. 
Sphcerexochus mirus (Fig. 
543) is almost a globe 
when rolled up, the fore¬ 
head or glabelliim of this 
species being extremely 
inflated. The Ilomalono- 
tus^ a form of Trilobite in 
which the tripartite divis¬ 
ion of the dorsal crust is 
almost lost (see Fig. 544), is very charac¬ 
teristic of this division of the Silurian series. 
'Wenloch Shale. —This, observes Sir R. 
^ou^^^^Wen- Murchison, is infinitely the largest and most 
Bocks Fudiow persistent member of the Wenlock forrna- 
tion, for the limestone often thins out and 
disappears. The shale, like the Lower Ludlow, often contains 
elliptical concretions of impure earthy 
limestone. In the Malvern district it is a 
mass of finely levigated argillaceous mat¬ 
ter, attaining, according to Professor Phil¬ 
lips, a thickness of 640 feet, but it is some¬ 
times more than 1000 feet thick in Wales, 
and is worked for flag-stones and slates. 
The prevailing fossils, besides corals and 
trilobites, and some crinoids, are several 
small species of Orthis^ Carcliola., and nu¬ 
merous thin-shelled species of Orthocera- 
tites. 
About six species of GraptoUte., a pecul¬ 
iar group of sertularian fossils before al- 
luded to (p. 463) as being confined to Silu- lock Limestone, Dud- 
rian rocks, occur in this shale. Of fossils Castie. 
of this genus, which is very characteristic of the Lower Silu- 
rian, I shall again speak in the 
^^^W^olhope‘Beds.—1\\oi\g\i 
Graptolithuspriodon,BvoYiM. Ludlow and not alwavS recognized aS a 
Wenlock shales. , 
separate subdivision ot the 
Wenlock, the Woolhope beds, which underlie the Wenlock 
shale, are of great importance. Usually they occur as massive 
or nodular limestones, underlaid by a fine shale or flag-stone; 
and in other cases, as in the noted Denbighshire sandstones, 
as a coarse grit of very great thickness. This grit forms 
mountain ranges through North and South Wales, and is 
generally marked by the great sterility of the soil where it 
