The cephalis is the most characteristic part of the shell of the #Botryodea# and its lobulate and multilocular shape separates them from the #Spyroidea# and #Cyrtoidea#. It represents the whole shell in the Cannobotryida, and in the young state of the two other families, which afterwards develop a thorax (Lithobotryida) and an abdomen (Pylobotryida). The typical lobulation of the cephalis is probably originally caused by internal septa; and these may be originated by branches of the internal columella, which corresponds either to the central ascending columella of the #Plectoidea# or to the excentric ascending dorsal rod of the sagittal ring of the #Stephoidea#. But afterwards, when the original septa disappear and are lost by reduction, only the external constrictions remain to indicate the limits of the single lobes. Among the internal septa and the corresponding external constrictions which effect the lobulation of the cephalis, may be distinguished primary or constant septa and secondary or inconstant septa. As primary septa we regard firstly an oblique frontal {1105}septum and secondly a vertical sagittal septum. The frontal septum (described accurately by Bütschli) has usually two or three pairs of pores and ascends obliquely from the base to the anterior face of the cephalis, where it is inserted in the orbital region; it divides the cephalis into a smaller anterior or facial lobe and a larger posterior or occipital lobe. The latter is commonly higher and more developed than the former and partly covers its upper face, like the crest of a helmet. The facial lobe is again bisected by an incomplete sagittal septum and a corresponding partial constriction on the anterior face. We find, therefore, in this original cephalis of the #Botryodea#, three primary lobes, an odd larger occipital lobe and two paired smaller facial lobes. The typical trilobed cephalis may be regarded perhaps as the common original form of the #Botryodea#.