Thecae
A frustule is usually composed of two identically shaped but slightly differently sized thecae. The theca which is a bit smaller has an edge which fits slightly inside the corresponding edge of the larger theca. This overlapping region is reinforced with silica girdle bands, and constitutes a natural "expansion joint." The larger theca is usually thought of as "upper," and is thus termed the epitheca. The smaller theca is usually thought of as "lower," and is thus called the hypotheca. As the diatom divides, each daughter retains one theca of the original frustule and produces one new theca. Interestingly, this means that one daughter cell is the same size as the parent (epitheca and new hypotheca) while in the other daughter the old hypotheca becomes the epitheca which together with a new and slightly smaller hypotheca comprises a smaller cell.
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