214 THE OLD RED SANDSTONE. 
ana the formation had a class of creatures in advance of the 
many-legged annelids of the other. It had its numerous family 
of trilobites, — crustaceans nearly as high in the scale as the 
common crab, — creatures with crescent-shaped heads, and 
jointed bodies, and wonderfully constructed eyes, which, like the 
eyes of the bee and the butterfly, had the cornea cut into facets 
resembling those of a multiplying glass. Is the reader ac- 
quainted with the form of the common Chiton of our shores 
— the little boat-shaped shell-fish, that adheres to stones and 
rocks like the limpet, but which differs from every variety of 
limpet, in bearing as its covering a jointed, nota continuous shell ? 
Suppose a chiton with two of its terminal joints cut away, anda 
single plate of much the same shape and size, but with two eyes 
near the centre, substituted instead, and the animal, in form at 
least, would be no longer a chiton, but a trilobite. There are 
appearances, too, which lead to the inference that the habits 
of the two families, though representing different orders of 
being, may not have been very unlike. The chiton attaches 
itself to the rock by a muscular sucker or foot, which, extend- 
ing ventrally along its entire length, resembles that of the 
slug or the snail, and enables it to crawl like them, but still 
more slowly, by a succession of adhesions. ‘The locomotive 
powers of the trilobite seem to have been little superior to 
those of the chiton. If furnished with legs at all, it must 
have been with soft rudimentary membranaceous legs, little 
fitted for walking with; and it seems quite as probable, from 
the peculiarly shaped under margin of its shell, formed, like 
that of the chiton, for adhering to flat surfaces, that, like the 
slug and the snail, it was unfurnished with legs of any kind, 
and crept on the abdomen. The vast conglomerations of 
trilobites for which the Silurian rocks are remarkable, are 
regarded as further evidence of a sedentary condition Like 
