OOLITIC SYSTEM OF SCOTLAND. 367 
which corresponds in this genus to the inclosing wall of Isas- 
trea, is very apparent in weathered specimens, but, as in 
Thamnastrea Lyellii, only faintly visible in those that are less 
worn ; while, as in the species Zhamnastrea scita, the colum- 
ella, if it at all possessed one, was rudimentary. I have 
usually found this species encrusting masses of indurated Old 
Red Sandstone of the flagstone formation, which must have 
been as ancient a looking rock in the times of the Oolite as it 
is now, and, when laid open by the waves along the beach, 
must have exhibited its ichthyolitic remains in their present 
state of keeping. In fine, in its rocks and stones this beach of 
the Oolite on what is now the eastern coast of Sutherland must 
have resembled that of the neighboring county of Caithness in 
the present day. And, as on the latter shore, as we approach 
the line of extreme ebb, we find rolled masses of dark gray 
flagstone, partially covered with pale-colored nulliporite en- 
crustations, there would have been found, had there been an 
inquiring eye to prosecute the search, similar dark gray masses, 
bearing their encrustations of Thamnastrea, along the old 
shores of the Oolite. 
But while the framework of the scenery must have been 
thus the same in both eras, and the same incalculably ancient 
sea must have broken in both against the same old fossil-bear- 
ing rocks, how entirely different must not the vital scenery of 
the two periods have been! Where we now see microscopic 
Lepralia and dwarfish Sertularia, huge Isastrea, embroidered 
by their flower-like polypes, and wide-spreading sheets of 
Thamnastrea, similarly mottled, must have gleamed white 
through the green depths of the water, as their existing repre- 
sentatives may be seen gleaming from the quiet recesses of 
tropical lagoons in the present day ; the ammonite and belem- 
nite must have careered over and around them amid the sheen 
of ganoidal scales ; and, where the seal now disports, the plesio- 
