The St. Cassian formation of northern Italy contains an unsurpassed diversity of Triassic organisms, often with original or only slightly recrystallized shells. The shell microstructure of Patella costulata Münster, 1869, is examined and compared with the shell microstructures of 14 extant gastropod limpet taxa. The distribution of shell structure characters supports the assignment of P. costulata to the patellogastropod taxon Patellidae. St. Cassian facies also include near-shore environments that are consistent with the habitat of extant Patella species. The corroboration of Patella costulata as a member of the Patellogastropoda reduces the hiatus between estimated origination and the previous unequivocal first occurrence by a minimum of 116 Ma.