A growth series of Stringocephalus gubiensis, described here, reveals that the cardinal process of Stringocephalus originated from a bilobate structure, and the growth of its dorsal median septum is highly related to cardinal process development. From this observation we conclude that Kaplex is, structurally and temporally, a suitable ancestor for Stringocephalus, with Bornhardtina ancestral to Kaplex. Subfamilial classification of the family Stringocephalidae is revised based on the presence or absence of median septa and the nature of hinge plates as well as on cardinal process formation. The Kaplexinae and Omoloninae are proposed as new subfamilies, with the Kaplexinae, including Kaplex and Erectocephalus, and the Omoloninae, including Omolonia, Kumbella, and Hemistringocephalus. Morphologic study of South China and Western Canadian Stringocephalus reveals that late forms of the genus tend to have a higher dorsal septum and more complex cardinal process than the early forms. In view of functional morphology, these trends reflect functional perfection of the cardinal process build-up and the lever system for improving the efficiency of opening and closing the shell.