THE OLD RED SANDSTONE. BES 
Ostree, Chitones, and other sedentary animals, they seemed 
to have adhered together in vast clusters, trilobite over trilo- 
bite, in the hollows of submarine precipices, or on the flat, 
muddy bottom below. And such were the master existences 
of three of the four Silurian platforms, and of the greater 
part of the fourth, if, indeed, we may not regard the cham- 
bered molluscs, their contemporaries, — creatures with their 
arms clustered round their heads, and with a nervous system 
composed of a mere knotted cord,—as equally high in the 
scale. Werise to the topmost layers of the system,— to 
an upper gallery of its highest platform, — and find nature 
mightily in advance. 
Another and superior order of existences had sprung into 
being at the fiat of the Creator— creatures with the brain 
lodged in the head, and the spinal cord enclosed ina vertebrated 
column. Inthe period of the Upper Silurian, fish properly so 
called, and of very perfect organization, had become denizens 
of the watery element, and had taken precedence of the crusta- 
cean, as, at a period long previous, the crustacean had taken pre- 
cedence of the annelid. In what form do these, the most ancient 
beings of their class, appear? As cartilaginous fishes of the 
higher order. Some of them were furnished with bony pal- 
ates, and squat, firmly-based teeth, well adapted for crushing 
the stone-cased zoophytes and shells of the period, fragments 
of which occur in their fecal remains ; some with teeth that, 
like those of the fossil sharks of the later formations, resem- 
ble lines of miniature pyramids, larger and smaller alternats 
ing ; some with teeth sharp, thin, and so deeply serrated that 
every individual tooth resembles a row of poniards set upright 
against the walls of an armory ; and these last, says Agassiz, 
furnished with weapons so murderous, must have been the 
pirates of the period. Some had their fins guarded with long 
