IV. Skeleton composed of two parallel horizontal rings (upper mitral and lower basal ring), both connected by a vertical sagittal ring (and often by a vertical frontal ring), 4. TYMPANIDA. Family XLVIII. #STEPHANIDA#, Haeckel (Pl. 81). _Monostephida_, Haeckel, 1881, Prodromus, p. 447. _Definition_.--#Stephoidea# with a simple sagittal ring, without any lattice-work. The family #Stephanida# is the most simple of all #Stephoidea#, and probably the common ancestral group of this suborder (compare above, p. 933). The skeleton {938}consists of a simple ring only, surrounding the central capsule, and armed commonly with simple thorns or with larger branched spines. The branches of these spines are constantly free, never joining together. Therefore the shell exhibits no trace of fenestration or lattice-work, no pores or gates, except the simple large gate of the ring itself. In all other #Stephoidea# we find secondary gates or lattice-work. The first known form of this family is _Lithocircus annularis_, described by Johannes Müller in 1858 (Abhandl. d. k. Akad. d. Wiss. Berlin, p. 29, Taf. i. fig. 1). He observed already the central capsule ("Blase") surrounded by the circular ring of silex, and the calymma ("Strahlige Gallert") enveloping the whole body. The peculiar structure of the soft body, particularly the "area porosa" on the basal pole of the central capsule, and the pseudopodial cone within it, were first accurately described by Hertwig (in 1879). A great number of simple rings, similar to these, were found in the Challenger collection, and are here arranged in six genera and forty-eight species. The simple ring of silex, which alone forms the skeleton of the Stephanida, corresponds to the primary or sagittal ring of the other #Stephoidea#, and lies therefore vertically in the median or sagittal plane of the body. We can therefore distinguish on it four different segments, rods or bows, two of which are more horizontal, two more vertical. The former are one upper bow or apical rod, and one lower bow or basal rod. The latter are one posterior bow or dorsal rod, and one anterior bow or ventral rod. Sometimes each of these four component bows is distinguished by a corner-spine, or by a pair of divergent branches.