ON THE IMPRESSIONS CONVEYED BY THE TERM TRANSITION. 277 
posed between the old red sandstone and the 
slaty-grauwacke of Wales. Hence it seemed 
very desirable to apply some distinctive name. 
So great have been the recent advances in 
geology, that the term “ transition,” formerly 
applied, has (as we observed above) now be- 
come wholly unmeaning, and, in fact, conveys 
incorrect impressions. Hence these geologists 
have adopted the name “ Silurian System,” 
(from the Roman name for this part of Wales;) 
and this they subdivide into the “ Upper Silu- 
rian,” comprising the two fii'st of the four 
classes named above, — and the “ Lower,” in- 
cluding the two last. 
Beneath these appear the various slaty 
rocks, which are common to Wales and Cum- 
berland. These Professor Sedgwick has 
minutely investigated ; and divides them, ac- 
cording to order of superposition, into upper, 
middle, and lower. The upper formation is 
seen in the chain of the Berwyn mountains, 
and is thence expanded over a large part of 
South Wales, including Plinlimmon ; it con- 
tains in general less calcareous matter, and 
fewer organic remains, than the Silurian 
systems. The middle Cambrian includes the 
Merionethshire ranges and Snowdon, con- 
taining a few organic remains, and some highly 
calcareous slates, but no beds of limestone. 
The same group is largely developed in Cum- 
berland. The lower or oldest Cambrian 
group occupies the south-west of Caernar- 
vonshire, and much of Anglesea. It con- 
tains no organic remains. 
In this rapid sketch, we, of course, can do 
little more than explain the nominal distinc- 
tions which have been thus laid down. But 
it must be understood, that they are far from 
being mere distinctions of names. They 
involve essential characteristics of extensive 
geological districts, and serve to bring under 
a luminous classification a series, of great 
importance to a connected knowledge of 
British strata, which has long been involved 
in obscurity from want of such a principle 
of arrangement. 
A full account of these researches was 
given at the Dublin meeting of the British 
Association ; and elicited besides the encomi- 
ums so justly due to the talents and persever- 
ance of its authors, many able illustrations 
and remarks; especially from Mr. Greenough, 
who considered that similar principles of 
classification might very probably be exten- 
ded to other regions ; and from Professor 
Phillips, who made some highly interesting 
observations on the distribution of charac- 
teristic organic remains in rocks, especially 
those here considered. He dwelt upon the 
important fact, so utterly destructive of the 
favourite hypothesis of some geologists and 
cosmogonists, of a gradual advance from the 
simplest to the most complex forms of animal 
life, as we advance to the newer rocks ; that 
in these Silurian groups, though we find a 
diminution in the number of fossil species in 
the older rocks, yet they exhibit no inferio- 
riiy of structure or organization. They 
belong to extinct classes. Among beings of 
lower organization, as among shell-fi.sh, some 
single species may be found even in rocks so 
ancient as the Silurian system, which also 
now exist; and he was hence led to remark, 
that it is not by any single genus, but by a com- 
bination of co-existing genera that strata must 
be identified. 
FOSSIL FISHES. 
The natural history of fishes has been gene- 
rally considered more obscure than that of 
any other of the great divisions of the animal 
kingdom; and it has been almost entirely 
through the labours of M. Agassiz that a 
new light has been thrown over it, by tracing 
out, as he has done, a new principle of clas- 
sification : by this the whole science has been 
remodelled, It is also a singular circumstance 
in this investigation, |;hat (contrary to the 
usual order of procedure) the study of the 
fossil remains of fishes has been a material 
source of elucidation for understanding the 
relations and classification of existing species. 
The great principle of classification 
adopted by M. Agassiz, is derived from the 
nature of the external covering or scales* The 
peculiar form and structure of the scales dif- 
fer essentially in different classes of fishes ; 
and the nature of the covering, which pro- 
tects the animal externally, is found to bear 
a direct relation to the internal organization. 
Here then there appears a principle of rela- 
tion which, doubtless, depends upon some 
essential modification of the animal cha- 
racter, and thus may fairly afford a satisfac- 
tory ground of a real distinction and classi- 
fication of species. This principle, then, M. 
Agassiz has adopted; and, in following it out, 
has arrived at a grand distinction of fishes, 
under four principal orders, characterized by 
the peculiar nature of their scales. .They are 
termed, 1. Placoi'dians, 2. Ganoidians, 3. 
Ctenoidians, and 4. Cycloi’dians 
Of the whole number of species now known 
to exist, more than three-fourths belong to 
the two orders of Cycloidians and Ctenoi'di- 
ans, the other fourth to the remaining two. 
Whereas, of the species whose fossil remains 
we find imbedded and mineralized, none of 
the two orders last-named have been found 
in any formations below the chalk ; whilst in 
the lower or older formations we have abund- 
ance of the other two kinds. The propor- 
tions of these in the different formations are 
very remarkable, and have been carefully 
traced by the persevering industry and skill 
of M. Agassiz, 
In the most recent or tertiary deposits, not 
only the fossil orders and genera, but also 
the species, approach nearly in character to 
* These names are derived from Greek 
words, describing the shape and appearance 
of the scales. 
1 . From plax, a table or broad surface, the 
scales being large. 
2. From ganos, beauty or splendour ; from 
the bright enamel with which they are armed. 
3. From cteis, a comb, the scales being 
formed with teeth. 
i. From cpclosy a circle, the scales being 
round. 
