|(Cyrtoidea monothalamia). | Archicorida. | Archicapsida. | +---------------------------+----------------+----------------+ | DICYRTIDA. | SETHOCYRTIDA. | |(Cyrtoidea dithalamia). | Sethocorida. | Sethocapsida. | +---------------------------+---------------------------------+ | TRICYRTIDA. | THEOCYRTIDA. | | (Cyrtoidea trithalamia). | Theocorida. | Theocapsida. | +---------------------------+----------------+----------------+ | STICHOCYRTIDA. | LITHOCAMPIDA. | |(Cyrtoidea polythalamia). | Stichocorida. | Stichocapsida. | +---------------------------+----------------+----------------+ {1130}The cephalis, or the first shell-joint of the #Cyrtoidea#, is in the majority homologous with the cephalis of the #Spyroidea#, from which it differs in the reduction of the sagittal ring and the absence of the corresponding sagittal constriction; its cavity is therefore simple, not bilocular. Its homology with the original cephalis of the #Spyroidea# cannot be doubted, when its base exhibits the typical basal pores of the Semantida. But in many cases these are wanting, and in a great number of #Cyrtoidea# (mainly of Monocyrtida) there is more or less evidence that the original cephalis is lost, and that the real first joint is the thorax, the original second joint. At present it is quite impossible to distinguish between the former and the latter shells, and therefore in the following descriptions the first joint is always named cephalis and the second thorax. In future, when the affinities of the #Cyrtoidea# become better known, it will be necessary to distinguish the "Archicephalis," or true cephalis of all #Spyroidea# and of the majority of #Cyrtoidea#, from the "Pseudocephalis" or the false cephalis of the minority (_e.g._, of many Monocyrtida aperta, Archipilida, Archiphormida, Archicorida, &c.).