BALA AND CARADOC BEDS. 
471 
There may, indeed, be said to be a continued decrease of 
the proportional number of this lower tribe of mollusca as 
we proceed from older to newer rocks. In the British De¬ 
vonian, for example, the Brachiopoda number 99, the Lamel- 
libranchiata 58 ; while in the Carboniferous their propor¬ 
tions are more than reversed, the Lamellibranchiata number¬ 
ing 334 species, and the Brachiopoda only 157. In the Sec¬ 
ondary or Cainozoic formations the preponderance of the 
higher grade of bivalves becomes more and more marked, till 
in the tertiary strata it approaches that observed in the liv¬ 
ing creation. 
While on this subject, it may be useful to the student to 
know that a Brachiopod dilfers from ordinary bivalves, mus¬ 
sels, cockles, etc., in being always equal-sided and never quite 
equi-valved; the form of each valve being symmetrical, it 
may be divided into two equal parts by a line drawn from 
the apex to the centre of the margin. 
Trilohites .—In the Bala and Caradoc beds the trilobites 
reach their maximum, being represented by 111 species re¬ 
ferred to 23 genera. 
Burmeister, in his work on the organization of trilobites, 
supposes that they swam at the surface of the water in the 
open sea and near coasts, feeding on smaller marine animals, 
and to have had the power of rolling themselves into a ball 
as a defense against injury. He was also of opinion that 
they underwent various transformations analogous to those 
of living crustaceans. M. Barrande, author of an admirable 
work on the Silurian rocks of Bohe.mia, confirms the doctrine 
of their metamorphosis, having traced more than twenty spe¬ 
cies through different stages of growth from the young state 
just after its escape from the egg to the adult form. He has 
followed some of them from a point in which they show no 
eyes, no joints, or body rings, and no distinct tail, up to the 
complete form with the full number of segments. This 
change is brought about before the animal has attained a 
tenth part of its full dimensions, and hence such minute and 
delicate specimens are rarely met with. Some of his figures 
of the metamorphoses of the common Trinucleus are copied 
in the annexed wood-cuts (Figs. 552, 553). It was not till 
1870 that Mr. Billings was enabled, by means of a specimen 
found in Canada, to prove that the trilobite was provided 
with eight legs. 
It has been ascertained that a great thickness of slaty and 
crystalline rocks of South Wales, as well as those of Snowdon 
and Bala, in North Wales, which were first supposed to be 
of older date than the Silurian sandstones and mudstones of 
