45 
of the forms may be only the young of others more mature. 
(Mr. Turner here enumerated a few of the most common genera 
pretty nearly in their order of time, some of which were illustrated 
by large drawings in the room). Many of the genera and specific 
names, he said, were arbitrary rather than descriptive, being 
derived sometimes from the discoverer, sometimes from the locality 
where first found or where abounding, and sometimes purely poetical. 
These animals had a very extensive range both in space 
and time. As regards space, the remains of the same genera are 
found in the palwozoic rocks of the Old World, America, and 
Australia; while as to time they may be traced through the 
immense palwozoic rocks from the Cambrian, in which they first 
appear, to the coal measures, beyond which, as yet, we find no 
trace of their existence. This represents a period of time con- 
cerning the length of which the human mind cannot form the 
faintest conception. How much earlier they may have existed, or 
how much later they may have continued, we at present do not 
know. But the trilobites are not equally distributed through the 
palwozoic rocks, either in space or time. They abound in some 
localities on the same level more than others, just as cockles and 
oysters, crabs and shrimps do now, their abundance or variety 
depending probably upon the plenty or scarcity of food and other 
circumstances. Some rocks literally swarm with their remains, 
whilst others of the same material, close by, are almost or quite 
barren. They appear, also, to have reached their maximum both 
in the number of genera and species in the upper beds of Lower 
Silurian rocks, after which they gradually died out in the coal 
measures. Moreover, the genera and species seem to have had 
their limited range, though often a long and wide range, in space 
and time. One cannot but be desirous to know what were the 
habits of these animals, their colour and form, whether they 
frequented deep or shallow water, and whether they were car- 
nivorous or vegetarians. Their remains are found most abundantly 
in mud-formed or limestone rocks, and less frequently, I think, in 
sandstone deposits, which seem to indicate that they lived at some 
distance from the shore, in water somewhat deep and still; or it may 
be that the sand did not preserve their remains so effectually as 
the mud. 
Below the lower Cambrian rocks—the Menevians of 
St. David’s—which yield the great Paradoxides, lie the still older 
Harlech grits and Barmouth sandstones, in which have been 
discovered, besides Paradoxites, Agnosti, and others, a genus of 
trilobite called Plutonia Sedgwickii, ‘‘ which” says Sir C. Lyell, 
‘is comparable in size to the large Paradoxides Davidis; has well 
developed eyes, and is covered with tubercles.” What are we to 
conclude from this? Are we to suppose that these large and well- 
