"Thalassiosira" have a cell wall made of silica, known as a frustule . The zygote is then becomes an auxospore, which has no rigid frustule. The common characteristic of diatom resting spores is a thick silica frustule. A frustule is the hard and porous cell wall or external layer of diatoms. A frustule is usually composed of two identically shaped but slightly differently sized thecae. The frustule forms as different sized valves, which overlap and are called thecae. For diatomea, the term in use is "frustule", and for radiolarians it should be "capsule".