wall or frustrule of diatoms isformed by two valves which are connected
to a ring of silica called a girdle. One valve and its girdle fits over
the other valve with its girdle. The outer valve is called the epitheca
and the inner one the hypotheca. A portion of each valve is bent at about
90 degrees where the valve is attached to its girdle, this portion
of the valve which is perpendicular to the rest of the valve is called
the valve mantle.