A third family of Prunoidea, closely allied to the two preceding families, is the Spongurida, in which we include all Prunoidea with an ellipsoidal or cylindrical, unjointed shell, in which the lattice-work of the cortical shell is transformed into an irregular, siliceous framework. In the simplest form, Spongellipsis, the simple lattice-shell of Cenellipsis is substituted by an external spongy envelope. In other cases (Spongurus and allied genera) the whole cavity of this external spongy shell is distended with a fine spongy framework. The subfamily of Spongodruppida is distinguished by the possession of a simple or double latticed medullary shell; this lies in the midst of the central capsule, and is connected by radial beams (perforating its membrane) with the enveloping spongy cortical shell. The surface of the latter may bear either radial spines, or two opposite strong polar spines, at the poles of the main axis (Pl. 17, fig. 12).
Closely allied to the Ellipsida and Druppulida are two other families of the Prunoidea, the Artiscida and Cyphinida, which differ from the former by a circular constriction in the equatorial plane of the ellipsoidal shell; and in this way assume a characteristic twin form, like a figure of eight. In the Artiscida the shell is simple (as in the Ellipsida), whereas in the Cyphinida it is composed of two or more concentric shells (as in the Druppulida). The simplest form of the Artiscida is Artiscus (Pl. 39, fig. 9), differing from Cenellipsis in the ring-shaped, equatorial constriction. In other Artiscida polar appendages are developed on both poles of the main axis, either in the form of solid, strong spines (Stylartus), or hollow fenestrated tubes (Cannartus, Pl. 39, fig. 10).
The family Cyphinida differs from the Druppulida in the equatorial constriction of the shell, and from the Artiscida in the presence of two or more concentric shells. One or two of these concentric fenestrated shells are enclosed in the central capsule (and therefore may be called "medullary shells"); the others (one or two, rarely more) lie outside of the central capsule (therefore "cortical shells"). The internal "medullary shells" are always spherical or somewhat lenticular, compressed from both sides; the external "cortical shells" have constantly a ring-like constriction in the equatorial plane, and "twin-shells" are therefore like a figure of eight. The simplest form of this subfamily is Cyphanta, composed of a simple medullary shell and a simple cortical shell, the two being connected in the equatorial plane by radial beams. In Cyphonium (Pl. 39, fig. 12) the medullary shell is doubled, and in Cypassis (Pl. 39, fig. 13) the cortical shell likewise. On both poles of the main axis strong spines are often developed (Cyphinus, Pl. 39, fig. 14), or hollow fenestrated tubes (Cannartidium, Pl. 39, figs. 16-19).
The equatorial constriction of the ellipsoidal shell, which characterises the Artiscida and Cyphinida, is repeated or multiplied in the two following families, in the Panartida and Zygartida; in the former we find three ring-like strictures, in the latter five or more (lying in parallel transverse planes); therefore the fenestrated shell is composed in the one instance of four chambers, in the other of six or
