UPPER LUDLOW BEDS. 
459 
UPPER SILURIAN^ ROCKS. 
1. Ludlow Formation.— This member of the Upper Silurian 
group, as will be seen by the above table, is of great thick¬ 
ness, and subdivided into two parts—the Upper Ludlow and 
the Lower Ludlow. Each of these may be distinguished near 
the town of Ludlow, and at other places in Shropshire and 
Herefordshire, by peculiar organic remains ; but out of more 
than 500 species found in the Ludlow formation as a whole, 
not more than five species per cent, are common to the over- 
lying Devonian. The student may refer to the excellent 
tables given in the last edition of Sir R. Murchison’s Siluria 
for a list of the organic remains of all classes distributed 
through the different subdivisions of the Upper and Lower 
Silurian. 
a. Upper Ludlow: Doionton Sandstoyie. —At the top of this 
subdivision there occur beds of fine-grained yellowish sand¬ 
stone and hard reddish grits which were formerly referred by 
Sir R. Murchison to the Old Red Sandstone, under the name 
of “ Tilestones.” In mineral character this group forms a 
transition from the Silurian to the Old Red Sandstone, the 
strata of both being conformable; but it is now ascertained 
that the fossils agree in great part specifically, and in gen¬ 
eral character entirely, with those of the underlying Upper 
Ludlow rocks. Among these are Orthoceras Indlatum^ Pla- 
tyschisma helicites^ BeUerophon trilohatus^ Choiietes lata^ etc., 
with numerous defenses of fishes. 
These beds, therefore, now generally called the “ Downton 
Sandstone,” are classed as the newest member of the Upper 
Silurian. They are well seen at Downton Castle, near Lud¬ 
low, where they are quarried for building, and at Kington, in 
Herefordshire. In the latter place, as well as at Ludlow, crus¬ 
taceans of the genera Pterygotus (for genus see Fig. 504, p. 
447) and Eurypterus are met with. 
JBo7ie-hed of the Upper Ludlow, —At the base of the Down- 
ton sandstones there occurs a bone-bed which deserves 
especial notice as afibrding the most ancient example of fos¬ 
sil fish occurring in any considerable quantity. It usually 
consists of one or two thin layers of brown bony fragments 
near the junction of the Old Red Sandstone and the Ludlow 
rocks, and was first observed by Sir R. Murchison near the 
town of Ludlow, where it is three or four inches thick. It 
has since been traced to a distance of 45 miles from that 
point into Gloucestershire and,other counties, and is common¬ 
ly not more than an inch thick, but varies to nearly a foot. 
Near Ludlow two bone-beds are observable, with 14 feet of 
