These constellations were arranged in three concentric annuli, the northern ones in an inner annulus subdivided into 60 degrees, the zodiacal ones into a medial annulus of 1 zo degrees, and the southern ones into an outer annulus of 240 degrees.
The section of a solid of revolution by a plane at right angles to the axis is an annulus or a sector of an annulus (fig.
The sporangium is of considerable size, and dehisces by a median slit, the annulus being a more or less definitely limited horizontal ring of cells near the apex.
In Stauropteris, a genus showing some affinity with Zygopteris, the branched rachis of the fertile frond terminates in fine branchlets, each bearing a single, spherical sporangium, without any differentiated annulus (fig.
There is, in birds, no annulus tympanicus.
The sporangia, which arise in basipetal succession on the receptacle, dehisce by a median slit, though the annulus is somewhat oblique; they have resemblances to the Gleicheniaceae.
Each has an almost horizontal annulus resembling that of Gleichenia, but the dehiscence is lateral.
The differences in the form and mode of dehiscence of the sporangia (those of the Simplices having median dehiscence and a horizontal annulus, those of the Gradatae a more or less oblique position of the annulus and of the plane of dehiscence, while in the Mixtae the annulus is vertical and the dehiscence transverse) stand in relation to the position of the sporangia in the sorus relatively to one another.
The sporangia dehisce by a transverse slit, the annulus being truly vertical or, in some of the genera in which they are regularly arranged, very slightly oblique.
The annulus is horizontal and the dehiscence median.