This study presents a taxonomic revision of the Alveolina elliptica group based on well-preserved specimens from the Sabzevar region of Central Iran. Detailed morphological and biometric analyses were conducted on key species, including A. kieli, A. stercusmuris, A. elliptica, and A. nuttalli. Our results document a wider morphological variability within A. stercusmuris than previously recognized and demonstrate that this species can be reliably distinguished from A. nuttalli by the absence of basal layer thickening in the equatorial plane, a diagnostic feature that is consistently developed in A. nuttalli. Ontogenetic variation observed in A. elliptica further indicates that several morphologies previously interpreted as taxonomically distinct instead reflect different growth stages. Based on a hierarchical evaluation of qualitative and semi-quantitative criteria, in which flosculinization represents the primary diagnostic character, followed by test shape and size, as well as coiling pattern and whorl development through ontogeny, this study confirms the taxonomic coherence of the Alveolina elliptica group and refines the diagnostic criteria used to distinguish its constituent species.
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