The order Billingsellida comprises brachiopods with laminar and fibrous shells, however, the data on their microstructure are scanty and the taxonomic value of differences in their shell structure is uncertain. The shell structure of clitambonitidine Antigonambonites planus was found to be laminar, not fibrous as previously considered; possibly this shows that it belongs to the suborder Billingsellidina. The pseudopunctae in the secondary layer normally are formed by deflected laminae and lack taleolae. Possible remains of the tertiary prismatic layer are described and the diagenetic changes are appreciated.
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