LOWER LUDLOW BEDS. 
461 
Pterinea^Cardiola^Ctenodonta (sub-genus of Nucula)^ Ortho- 
nota^ Modiolopsis^ and Pcdcearca. 
Some of the Upper Ludlow sandstones are ripple-marked, 
thus affording evidence of gradual deposition; and the same 
may be said of the accomi^anying fine argillaceous shales, 
which are of great thickness, and have been provincially 
named “ mud-stones.” In some of these shales stems of cri- 
noidea are found in an erect position, having evidently be¬ 
come fossil on the spots where they grew at the bottom of 
the sea. The facility with which these rocks, when exposed 
to the weather, are resolved into mud, proves that, notwith¬ 
standing their antiquity, they are nearly in the state in which 
they were first thrown down. 
Lower Ludlow Beds. — The chief mass of this formation 
consists of a dark gray argillaceous shale with calcareous 
concretions, having a maximum thickness of 1000 feet. In 
some places, and especially at Aymestry, in Herefordshire, a 
subcrystalline and argillaceous limestone, sometimes 50 feet 
thick, overlies the shale. Sir R. Murchison therefore classes 
this Aymestry limestone as holding an intermediate posi¬ 
tion between the Upper and Lower Ludlow, but Mr. Light- 
body remarks that at Mocktrie, near Leintwardine, the Lower 
Ludlow shales, with their characteristic fossils, occur both 
above and below a similar limestone. This limestone around 
Aymestry and Sedgeley is distinguished by the abundance of 
Pentamerus Knightii^ Sow. (Fig. 529), also found in the Low- 
rig. 529. 
Pentamerus Knightii, Sow. Aymestry. One-half natural size. 
a. View of both valves united, h. Longitudinal section through both valves, 
showing the central plates or septa. 
er Ludlow and Wenlock shale. This genus of brachiopoda 
was first found in Silurian strata, and is exclusively a palae¬ 
ozoic form. The name was derived from wevre, pente^ five, 
and /xejooc, meros^ a part, because both valves are divided by 
a central septum, making four chambers, and in one valve 
