2. Wenlock Formation.—We next come to the Wenlock formation, which has been divided (see Table, [ p. 458]) into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.

a. Wenlock Limestone.—This limestone, otherwise well known to collectors by the name of the Dudley Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. This ridgy prominence is due to the solidity of the rock, and to the softness of the shales above and below it. Near Wenlock it consists of thick masses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed “ball-stones” in Shropshire, are often enormous, even 80 feet in diameter. They are of pure carbonate of lime, the surrounding rock being more or less argillaceous[[2]] Sometimes in the Malvern Hills this limestone, according to Professor Phillips, is oolitic.

Among the corals, in which this formation is so rich, 53 species being known, the “chain-coral,” Halysites catenularius (Fig. 536), may be pointed out as one very easily recognised, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to near the bottom of the Llandeilo rocks. Another coral, the Favosites Gothlandica (Fig. 537), is also met with in profusion in large hemispherical masses, which break up into columnar and prismatic fragments, like that here figured (Fig. 537, b). Another common form in the Wenlock limestone is the Omphyma turbinatum (Fig. 538), which, like many of its modern companions, reminds us of some cup-corals; but all the Silurian genera belong to the palæozoic type before mentioned ([p. 432]), exhibiting the quadripartite arrangement of the septalamellæ within the cup.

Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus see [Figs. 478], 479) contribute their calcareous stems, arms, and cups towards the composition of the Wenlock limestone. Of Cystideans there are a few very remarkable forms, most of them peculiar to the Upper Silurian formation, as, for example, the Pseudocrinites, which was furnished with pinnated fixed arms,[[3]] as represented in Fig. 539.

The Brachiopoda are, many of them, of the same species as those of the Aymestry limestone; as, for example, Atrypa reticularis (Fig. [532]), and Strophomena depressa (Fig. 540); but the latter species ranges also from the Ludlow rocks, through the Wenlock shale, to the Caradoc Sandstone.