The Orthoceras Ludense ([fig. 417.]), as well as the shell last mentioned, is peculiar to this member of the series. The Homalonotus delphinocephalus ([fig. 418.]) is common to this division and to the Wenlock limestone. This crustacean belongs to a group of trilobites which has been met with in the Silurian rocks only, and in which the tripartite character of the dorsal crust is almost lost.

Fig. 418.

Homalonotus delphinocephalus, König.[354-A] Dudley Castle; 1/2 nat. size.

A species of Graptolite, G. Ludensis, Murch. ([fig. 419.]), a form of zoophyte which has not yet been met with in strata newer than the Silurian, occurs in the Lower Ludlow.

Wenlock formation.—We next come to the Wenlock formation, which has been divided (see Table, [p. 351.]) into

1. Wenlock limestone, formerly well known to collectors by the name of the Dudley limestone, which forms a continuous ridge, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. The prominence of this rock in Shropshire, like that of Aymestry, is due to its solidity, and to the softness of the shales above and below. It is divided into large concretional masses of pure limestone, and abounds in trilobites, among which the prevailing species are Phacops caudatus ([fig. 422.]) and Calymene Blumenbachii, commonly called the Dudley trilobite. The latter is often found coiled up like a wood-louse (see [fig. 420.]).