
678 - STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
thin-shelled forms of Orthoceras are specially abundant. Among the 
trilobites, Encrinurus punctatus, E. variolaris, Calymene Blumenbachii, C. 
tuberculosa, Phacops caudatus, and P. longicaudatus are common. The 
Monograptus (Graptolithus) priodon, so frequent among the Bala beds of 
the Lower Silurian series, also occurs in the Wenlock shale; while 
M. (Graptolithus) Flemingii is here a characteristic species. 
(d.) Wenlock Limestone.—This is a thick-bedded, sometimes flaggy, 
usually more or less concretionary limestone, grey or pale pink, often 
highly crystalline, occurring in some places as a single massive bed, 
in others as two or more strata separated by grey shales, the whole 
forming a thickness of rock ranging from 100 to 300 feet. As its 
name denotes, this zone is typically developed along Wenlock Hdge 
in Shropshire, where it runs as a prominent ridge for fully 20 
miles; also between Aymestry and Ludlow. It lkewise appears at 
the detached areas of Upper Silurian strata above referred to, being 
specially well seen near Dudley (whence it is often spoken of as 
the Dudley limestone), Woolhope, Malvern, May Hill, and Usk in 
Monmouthshire. 
A distinguishing characteristic of the Wenlock limestone is the 
abundance and variety of its corals, of which no fewer than 25 genera 
and 76 species have been described, of which 41 species are peculiar to 
the Wenlock group. The rock seems indeed to have been formed in 
part by massive sheets and bunches of coral. Characteristic species are 
Halysites catenularia, Heliolites interstinctus, H. tubulatus, Alveolites Labechet, 
Favosites aspera, F. fibrosa, F'. gothlundica, Cenites juniperinus, Syringopora 
fascicularis, and Omphyma turbinatum. The ecrinoids are also specially 
abundant, and often beautifully preserved: 20 genera make their first 
appearance in the Wenlock group, and 17 are confined to it, among — 
the 65 species which have been named, Periechocrinus moniliformis 1s one 
of the most frequent ; others being Crotalocrinus rugosus, Cyathocrinus gonio- 
dactylus, and Marsupiocrinus celatus. Several cystideans occur, of which one 
is Pseudocrinites quadrifasciatus. 'The annelides number 34 species. The 
crustaceans include numerous trilobites, among which we miss some of the 
persistent Lower Silurian genera, suchas Asaphus, Ogygia, and Trinucleus, 
none of which ascend into the Wenlock group. The most abundant 
trilobite is the long-lived Calymene Blumenbachii, which ranges from the 
Llandeilo flags up to near the top of the Upper Silurian formations. It 
occurs abundantly at Dudley, where it received the name of the “ Dudley 
Locust.” Other common forms are Encrinurus punctatus, EH. variolaris, 
Phacops caudatus, P. Downingize, P. Stokesiit, Bumastus Barriensis, Homalo- 
notus delphinocephalus, and Cheirurus bimucronatus. One of the most 
remarkable features in the crustaceous fauna is the first appearance of 
the merostomata, which are represented by Lurypterus punctatus, Hemiaspis 
horridus, and Pterygotus problematicus. he brachiopods continue to be 
abundant, 21 genera and 96 ‘species having up to this time been enu- 
merated; among typical species may be noted Atrypa reticularis, Meri- 
stella tumida, Spirifera elevata, S. plicatella, Rhynchonella borealis (very 
common), Lt. cuneata, Lt. Wilsoni, Orthis elegantula, O. rustica, Strophomena 
rhomboidalis, and Pentamerus galeatus. 'Che lamellibranchs are represented 
by 43 species; among these several species of Pterinea, Cardiola, and 
Cucullella are abundant, with Grammysia cingulata, and some species of 
Modiolopsis aud Ctenodonta, 'The gasteropods are marked by species 
