ROBERTS — DISTRrBUTION OF CEPHALASPIS AND PTERASPIS. 189 
DISTRIBUTION OF CEPHALASPIS AND PTERASPIS. 
(Geol. vol. IV., p. 102 ; Ihicl, p. 140; 
Sir, — Before I trouble you with a few remarks in elucidation of the 
apparent difference between the ^aews of Mr. Lightbody and my own, 
permit me to express the pleasure I feel at seeing his name so promi- 
nently in your columns. No man has studied with such untiring 
zeal the range and sequence of those interesting deposits which make 
up the Ludlow promontory ; and I am glad to find that my own 
investigations, carried on independently of his, and at a distance 
from the field of work, have provoked so slight a bill of exceptions, 
and a few interesting notes, which I am sure every reader of " The 
Geologist" would be glad to see continued. 
The head and front of my offending, as I learn it from my friend's 
comments is this: — I have called the "Passage-beds" Lower 
Tilestones ; and out of this some confusion has arisen in minds 
which associate the word " tilestones," upper or lower, with those 
originally so-called, but known by Sir Roderick Murchison now as 
the Downton-beds. To make the position of Mr. Lightbody and 
myself quite clear to your readers, I will refer them to my section on 
p. 104. They will see that above the " Downton Sandstones" lies a 
zone which I have called " Lower Tilestones," and which is marked 
out by the number and variety of its fish-fossils. These beds are the 
"Passage-shales" of Murchison, which have been so industriously 
worked in their exposures in the Ludlow district (the chief of which 
are in the railway cutting near the station, and at the Tin Mill, about 
a mile distant) b}^ my valued friend, Lightbody. 
But now followeth my reason for not, in my humble sketch of 
ancient ichthyic life, retaining a name which has such high sanction. 
I rejected the term because it appeared to me to have less value as 
a designation for a special zone of deposit than the one I employed. 
The horizon of a "Passage-bed" must necessarily, from the character 
of the powers employed to deposit it, be a shifting one. True, that 
no name or term of designation we can apply to any rock, or zone of 
deposit, will be cosmopolitan in its value ; but " Passage-beds" so 
called, have more troublesome equivalents than deposits nearer to the 
centre of a system, and in any endeavour to sketch out the range of 
life-remains, the term seemed to me peculiarly inappropriate. Upon 
this view of the case, I included n\\ the beds beneath the " Upper 
Tilestones" of Trimpley — and as I still think of the Downton Hall 
drive quarry^ — under one name, as Lower Tilestones, representing 
them as resting upon the Downton series of sandy and " tiley" rocks. 
One good characteristic of the "Upper Tilestones" is their possession 
of an intercalated plant -bed, w4th good evidences of terrestrial vege- 
tation. This, it must be remembered, is quite a distinct thing from 
ihe Downton plant-bed, which contains the earliest land-plants. The 
