93 THE MEDALS OF CREATION. Cuap. VI. 
openings, composed of pure silex, and in many species — 
is exquisitely ornamented. Figs. 8 and 9. show a living © 
Navicula, viewed in front, and in profile: in jig. 9 are 
represented the currents produced when the body is moy- 
ing through the water ; after Ehrenberg. Fossil Vavicule 
abound in many tertiary strata. 
Galionella. .Plate IV. jigs. 10, 11.—These algze are 
free, and the frustules of a cylindrical, globular, or discoidal 
form ; they occur in chains, in consequence of the self- 
division being imperfect, and the new individuals remain- — 
ing attached to the old. The Galionelle are most abundant 
and prolific, and inhabit every pool, stream, and lake: 
fossil species occur in the Virginian marls, and other 
strata. 
Synhedra. Plate IV. fig. 12.—The frustules are siliceous, — 
and of a slender, elongated form. ‘The plant is attached by 
the base (jig. 12a.) in youth, and afterwards becomes free. 
t is found fossil in the Mountain-meal of Santa Fiora, and — 
many other deposits. 
Podosphenia. Plate IV. fig. 13.—The frustule is eruci- 
form, or wedge-shaped, and attached in youth by the small 
end, but afterwards becomes free. These plants are often 
arranged in clusters, as in the figure. M. Ehrenberg © 
states that they inhabit the sea, and not fresh-water ; but 
I have found them in streams communicating with the 
Thames. Podosphenice abound in the polishing slate of 
Bilin. : 
Eunotia. Plate IV. figs. 16, 17.—The frustule is sili- 
ceous, and either simple or bivalve ; flat below, and convex, 
and often richly dentated above. An empty case is shown 
jig. 16; and a group of living Hunotie attached to a stem 
of conferva, fig. 17. Several fossil species have been dis- 
covered at Santa Fiora. 
That the general reader, whose attention is for the first 
time directed to this subject, may be prepared for the enor- 
