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.).
The thorax, or the second shell-joint of the Cyrtoidea, is in the majority homologous with the thorax of the Phormospyrida and Androspyrida, and therefore developed by apophyses, which arise from the base of the cephalis and become united by transverse branches forming a lattice-plate. Its size is generally in inverse proportion to that of the cephalis. The more the cephalis becomes reduced, the more the thorax is developed. Its form is very variable, usually three-sided pyramidal or prismatic in the triradiate, polyhedral in the multiradiate, and conical or cylindrical in the eradiate Cyrtoidea. Its terminal mouth is either a simple wide opening, or closed by a lattice-plate. In the majority of Cyrtoidea the thorax is separated from the cephalis not only by the external collar constriction, but also by the internal cortinar septum, a horizontal lattice-plate which exhibits the typical basal pores of the Semantida (usually two smaller jugular and two larger cardinal pores). But this septum is often reduced or perfectly lost, and then the external collar constriction alone indicates the separation of the cephalis and the thorax.
The abdomen, or the third shell-joint of the Cyrtoidea, absent in the Monocyrtida and Dicyrtida (as also in all Spyroidea), occurs constantly in all Tricyrtida and Stichocyrtida. It is a simple large chamber in the Tricyrtida, but forms an annulated body, composed of a variable number of successive joints, in the Stichocyrtida. The constrictions between these joints, and also the lumbar constriction, between abdomen and thorax, are usually provided with a lattice-girdle, projecting into the cavity of the shell, like a diaphragm. Usually this horizontal girdle bears only a single circle of pores, rarely two or more. In many Cyrtoidea it is replaced by a solid horizontal ring of silex, and often it is wanting. It originates by the insertion of the following shell-joint, which takes place not on the terminal mouth of the preceding joint, but somewhat above it.
