Chap. VII.] 
UPPER LUDLOW BONE-BED. 
141 
In this rock there is, indeed, a form of Fish not yet seen in the inferior 
courses, to which Sir P. Egerton assigned in 1857 the name of Cephalaspis 
Fossils (23) of the Uppermost Bone-bed, near Ludlow. 
3 2 
1. Cephalaspis Murchi- 
soni, Egerton. Inside 
of head-shield. Less 
than half the natural 
size. 
Murchisoni (Quart. Journ. Geol. Soc. vol. xiii. p. 284). It is represented in 
the preceding woodcut ; and differing, as it does, in a marked manner, by 
the truncated, not arched, posterior margin, from Cephalaspis Lyellii of 
the Old Red Sandstone, this species may be regarded as characteristic of a 
bed which indicates a true passage from the summit of the Silurian rocks 
into the Devonian rocks or Old Red Sandstone. In the copious develop- 
ment of red marls and thick-bedded sandstones which follows, the fossils 
that characterize the inferior strata are no longer detected. 
Another example of the ascending order from the mass of the Upper 
Ludlow rock, through the chief bone-bed, into the overlying Downton 
Castle building-stone, has been detected by Mr. Alfred Marston, near 
Norton, on the road to Onibury, where the fish-bed, six inches thick, is 
surmounted by a thin course charged with Pterygoti, and covered by 
argillaceous layers (replete with Platychisma helicites and Beyrichia 
Kloedeni) which form the base of the Downton Castle sandstone. Mr. 
Lightbody has observed a similar succession at the north side of Whit- 
cliffe Coppice, near Ludlow, as well as in the section shown by the Lud- 
low Railway-cutting. 
The persistence of the uppermost band of the Ludlow rock, with its 
land-plants, fishes, and the same species of shells, at considerable distances 
from the tract in which it was first described, is truly remarkable, 
and shows the value of close and minute researches over extensive 
areas. 
The Tilestones or Passage-beds are visible all along the eastern frontier 
of the Silurian rocks (particularly from Kington to the Trewerne Hills on 
the Wye), and scarcely exceed forty or fifty feet of maximum thickness, 
including both their thick and thin strata and even some reddish marl and 
micaceous sandstones. They constitute, therefore, both lithologically and 
zoologically, a true transition, and cannot be arbitrarily classed as a 
whole either with Silurian or Devonian. It is, however, manifest from 
