here one main axis is constantly larger than all other axes (Monaxonia). Usually both poles of this main axis are equal (Haplopola); but in some genera both poles become different (Diplopola).
In the Sphæroidea all planes going through the centre of the shell are circular, whereas in the Prunoidea only those planes are circular which are perpendicular to the main axis; all other planes going through the centre are elliptical; the largest of these are the meridian planes, in which is situated the main axis. Commonly all meridian planes are equal, as no transverse axes (or cross axes) are differentiated.
In my Monograph (1862) only very few forms of Prunoidea, such as Didymocyrtis and Spongurus, are described, and the greater part of them are distributed under different genera of Sphæroidea, such as Haliomma and Actinomma. In my Prodromus (1881) I separated them as the family Zygartida (p. 462). But it seems now much more convenient to restrict this term to a particular family and to give a wider extension to the whole suborder under the name Prunoidea (called after the characteristic ellipsoidal form of a plum, or Prunus, with its stone).
The suborder Prunoidea comprises seven different families, of which the Ellipsida constitutes the simplest and the probable common ancestral group. In all Ellipsida the fenestrated shell is simple, and never composed of concentric or twin shells. In their primitive genus Cenellipsis, the whole shell is geometrically nothing more than a simple ellipsoid (Pl. 39, figs. 1, 2). By development of radial spines it passes into Ellipsidium, by development of spongy framework into Spongellipsis. In the greater part of this family large spines are developed on both poles of the main axis of the shell, but sometimes instead of these solid spines, two opposite hollow fenestrated tubes are developed (Pipettella, Pl. 39, fig. 6).
The second family of Prunoidea, the Druppulida, is much richer in different forms than the simple Ellipsida. In this case the ellipsoidal shell is composed of two or three (rarely more) concentric shells. Constantly one or two of these fenestrated shells are enclosed in the central capsule, and may therefore be called "medullary shells"; and one or two (rarely more) lie outside the central capsule, "cortical shells." The inner medullary shells (one or two) are either spherical or ellipsoidal; the outer cortical shells (one or two, rarely more) are always ellipsoidal. All concentric shells are connected by radial beams. In the simplest form of the subfamily, Druppula (Pl. 39, fig. 3), one medullary shell is connected with one cortical shell. By duplication of the medullary shell arises Prunulum (Pl. 39, fig. 4), and by duplication or multiplication of the cortical shell Cromyodruppa (Pl. 15, figs. 1-4) is formed. In by far the greater portion of this subfamily large spines are developed on both poles of the main axis of the cortical shell (Pls. 16, 17), but sometimes also instead of these solid spines two opposite hollow fenestrated tubes are developed (Pipetta, Pl. 39, figs. 7, 8).
