466 
ELEMENTS OF GEOLOGY. 
Fig. 539. 
Fig. 538. Wenlock limestone is the Omphyma turhi- 
natiim (Fig. 538), which, like many of its 
modern companions, reminds us of some 
cup-corals; but all the Silu¬ 
rian genera belong to the 
palteozoic type before men¬ 
tioned (p. 432), exhibiting 
the quadripartite arrange¬ 
ment of the septalamellse 
within the cup. 
Among the numerous 
Crinoids, several peculiar 
Omphyma turbinatuni, specieS of (Jl/OthoCTiTltlS (lor 
Linn. sp. {Cyatho- genus, See Figs. 478, 479, 
WenSLMeSe! P* Contribute their cal- 
Shropshire. careous Stems, arms, and 
cups towards the composition of the Wen- lock Limestone, 
lock limestone. Of Cystideans there are a 
few very remarkable forms, most of them peculiar to the 
Upper Silurian formation, as, for example, the Pseudoerinites^ 
which was furnished with pinnated fixed arms,^ as represent¬ 
ed in the annexed figure (Fig. 539). 
The Brachiopoda are, many of them, of the same species as 
those of the Aymestry limestone; as, 
for example, Atrypa reticidaris (Fig. 
532, p. 462), and Strophomena de- 
pressa (Fig. 540); but the latter spe¬ 
cies ranges also from the Ludlow 
rocks, through the 
Wenlock shale, to 
^ , the Caradoc Sand- 
StropJiomena (Leptcena) oepres- 
sa, Sow. Wenlock and Lud- Stone, 
Crustaceans 
are represented almost exclusively by Tri- 
lobites, which are very conspicuous, 22 be- ^ .... 
mg peculiar. Ihe Calymene JJmme^ioacmt Brong. Ludlow, Weu- 
(Fig. 541), called the ‘'Dudley Trilobite,” lock, and Bala beds, 
was known to collectors long before its true place in the ani¬ 
mal kingdom was ascertained. It is often found coiled up 
like the common Oniscus or wood-louse, and this is so usual 
a circumstance among certain genera of trilobites as to lead 
us to conclude that they must have habitually resorted to 
this mode of protecting themselves when alarmed. The other 
common species is the Phacops caudatus {Asaphus caMcIatus)^ 
Brong. (see Fig. 542), and this is conspicuous for its large 
* E. Forbes, Mem. Geol. Survey, vol. ii., p. 496. 
Fig. 540. 
Fig. 541. 
