124. _The Circoid Skeleton._--This is a very important and remarkable type of skeletal formation, which occurs exclusively in the legion NASSELLARIA, where it plays a very prominent part; its characteristic element is the "sagittal ring," a simple, vertical, siliceous ring, which surrounds the central capsule in its sagittal plane, and is specially differentiated in its basal portion. This "primary sagittal ring" whose vertical allopolar main axis coincides with that of the Monopylean central capsule embraced by it, is characteristic of all members of the order #Stephoidea# (p. 931, Pls. 81-83, 92-94); here it forms by itself the skeleton of the Stephanida (Pl. 81); in the Semantida (Pl. 92) it is combined with a horizontal basal ring, in the Coronida (Pls. 82, 93) with a vertical frontal ring and in the Tympanida (Pls. 83, 94) with two horizontal rings, an upper mitral and a lower basal. In the great majority of these #Stephoidea# there often develop in definite places characteristic processes or apophyses, whose branches combine to form a loose tissue or an incomplete lattice-shell. This becomes complete in the #Cyrtellaria#, the majority of which retain more or less {lxxxiii}distinct traces of the sagittal ring. Hence the skeletons of all NASSELLARIA may be derived monophyletically (Hypothesis A, p. 893) from a simple sagittal ring (_Archicircus_ and _Lithocircus_, Pl. 81). This theory, however, encounters the great difficulty that in many #Stephoidea# (_Cortina_, _Cortiniscus_, &c.) it is combined in a remarkable manner with the basal tripod of the #Plectoidea#, whilst in these latter it is entirely wanting (compare p. 894).