What's the difference between frustule and frustum?

Frustule

Definition:

(n.) The siliceous shell of a diatom. It is composed of two valves, one overlapping the other, like a pill box and its cover.

Example Sentences:

(1) The species-specific, genetically determined silica frustule of diatoms provides an excellent eukaryotic model for understanding the process of biomineralization and how it is regulated.
(2) An effort is made to define molecular events and biochemical constituents in silicon mineralization and to correlate these events with those aspects of silica frustule morphogenesis which have been reported in the published work.

Definition:

(n.) The part of a solid next the base, formed by cutting off the, top; or the part of any solid, as of a cone, pyramid, etc., between two planes, which may be either parallel or inclined to each other.
(n.) One of the drums of the shaft of a column.

Example Sentences:

(1) A new mitral valve replacement, in a truncated cone or frustum configuration is being evaluated, utilizing this test facility.
(2) This empirically derived formula differs from previous equations in that: the thoracic volume conductor is modeled geometrically as a frustum (truncated cone), rather than a cylinder; the specific resistivity of blood, rho, is appropriately eliminated as an independent variable; the equation employs a percentage of overall body height in lieu of a measured thoracic length; and a new term, delta, is introduced to scale the parent equation for deviation from ideal body weight.
(3) The large subunit has the shape of a conical frustum, and has a groove at its flat side.
(4) The cross-sectional shape of the S ring is that of a frustum rather than a disk.
(5) Thereafter, the total and necrotic volume were calculated assuming that the ventricles form a cone-like structure composed of successive frustum of a cone crowned by an apical cone.