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Diagnosis: Large, moderately thick-walled spherical shell, with circular to subcircular (rarely polygonal), small pores, 2-3 times thickness of bars, sometimes incompletely framed, with very numerous, long (about 2 times pore diameter) by-spines. Mouth small, circular, with 5-7 (rarely 3-10) uniform, robust, conical, long (twice length of by-spines), convergent teeth.
Three subspecies were distinguished by Haecker (1908) and one by Tibbs (1969), and two forms were recognized by Kling (1966), based mainly on size and wall thickness. Polar forms are generally larger.

Diameter: 0.3-1.1 mm.

Castanella thomsoni