680  STRATIGRAPHICAL GEOLOGY. [Book VI. 
of Euomphalus, Murchisonia, Holopella, Pleurotomaria, Acroculia, Cyclonema. 
The cephalopods are confined to five genera, Lituites, Actinoceras, Cyrtoceras, 
Orthoceras, and Phragmoceras; of these the orthoceratites are by far 
the most abundant both in species and individuals. Orthoceras annulatum 
is the most common form. The pteropods appear in the beautiful and 
very abundant Oonularia Sowerbyi, and the heteropods in. the common 
and characteristic Bellerophon Wenlockensis. 
3. Ludlow Group.—This series of stata consists essentially of shales, 
with occasionally a calcareous band in the middle. It graduates down- 
ward into the Wenlock group, so that when the Wenlock limestone 
disappears, the Wenlock and Ludlow shales form one continuous 
argillaceous formation, as they do where they stretch to the south- 
west through Brecon and Carmarthen. ‘The Ludlow rocks, typically 
seen between Ludlow and Aymestry, appear likewise at the detached 
Silurian areas from Dudley to the mouth of the Severn. They 
were grouped by Murchison intojthree zones. Their fauna numbers at 
present nearly 400 species, of which 129 are also found in the Wenlock 
roup. 
; es Lower Ludlow Rock.—This is a group of soft dark-grey to pale 
greenish-brown or olive sandy shales, often with calcareous concretions. 
Much of the rock, however, presents so little fissile structure as to get 
the name of mudstone, weathering out into concretions which fall to 
angular fragments as the rock crumbles down. It becomes more sandy 
and flagey towards the top. From the softness of the shales this zone of 
rock has been extensively denuded, and the Wenlock limestone rises up 
boldly from under it. 
An abundant suite of fossils has been yielded by these shales. 
Eight species of star-fishes, belonging to the genera Protaster (like the 
brittle-stars of the British seas), Palzodiscus, and Paleocoma. A few 
graptolites (eight species belonging to Graptolithus or Monograptus) occur, 
particularly the persistent Monograptus (Graptolithus) priodon (common), 
M. colonus, and M. Flemingit. A few corals occur in the Lower Ludlow 
rock, all of species that had already appeared in the Wenlock limestone, 
but the conditions of deposit were evidently unfavourable for their 
growth. The trilobites are less numerous than in older beds; they in- 
clude the venerable Calymene Blumenbachti, Phacops caudatus, and its still 
longer-tailed variety P. longicaudatus ; also Acidaspis Brightii, Homalonotus 
delphinocephalus, and Cyphaspis megalops. But other forms of crustacean 
life occur in some number. As the trilobites begin to wane, numerous 
phyllopods appear, the genus Ceratiocaris being represented by ten or 
more species. Still more remarkable, however, was the increasing im- 
portance of the merostomatous crustaceans. Though brachiopods are 
not scarce, hardly any seem to be peculiar to the Lower Ludlow rock; 
of the 38 known species 33 occur in the Wenlock group. Rhynchonella 
Wilsoni, Spirifera exporecta, Strophomena euglypha, Atrypa reticularis, and 
Chonetes minima are not infrequent. Among the more frequently recurring ~ 
species of lamellibranchs the following may be named—Cardiola interrupta, 
C. striata, Orthonota rigida, O. semisulcata, and a number of species of 
Ptermea. 'Vhe orthoceratites are numerous, as Orthoceras Ludense, O. 
subundulatum, also species of Phragmoceras and Lituites. The numbers of — 
these straight and curved cephalopods form one of the distinguishing 
features of the zone. At one locality, near Leintwardine in Shropshire, 

