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 more; all the chambers form a single series and have a common main axis. All constrictions lie in planes parallel to the equatorial plane of the original ellipsoid; in the centre of the latter constantly lies a double "medullary shell," composed of two concentric, either spherical or lenticular, compressed shells. In all Panartida we call the two inner chambers (on both sides of the equatorial constriction) "proximal chambers," the two outer chambers (on the poles of the main axis) "distal chambers." The four-chambered cortical shell of the Panartida is either simple (in Panartus, Pl. [40], figs. 1-4) or double, with an external mantle (as in Peripanartus, Pl. [40], figs. 5-7). The simplest form of the subfamily is Panartus (loc. cit.). In this case also on both poles of the main axis may be developed solid spines, or hollow fenestrated tubes (Panarium, Pl. [40], fig. 9).
The seventh and last family of the Prunoidea, the Zygartida is most nearly allied to the Panartida, and appears as a further developmental step from that family. Whilst in the Panartida the cortical shell is constantly four-chambered, with three parallel ring-like constrictions, in the Zygartida it is always prolonged and composed of six or more chambers, separated by five or more ring-shaped constrictions, in the middle of which is the equatorial stricture. In the centre of the latter (as also in the Panartida) always lies the double medullary shell, composed of two concentric, spherical, or lenticular shells. The number of the chambers of the cortical shells is commonly six or eight (with five to seven ring strictures), but it often mounts to ten and sometimes to twenty (with nineteen strictures), as in some species of Zygartus (Pl. [40], fig. 13). All the chambers lie in one series, one behind another, with a common main axis. The cortical shell is usually simple (in Ommatocampe, Pl. [40], fig. 10), sometimes double (in Desmocampe, Pl. [40], fig. 12), rarely triple (in Zygocampe, Pl. [40], fig. 13). In all three cases hollow fenestrated tubes may be developed on the poles of the main axis.
The morphological references and the phylogenetic affinities of all Prunoidea are so complex, that they seem to represent a quite natural group; all forms of it may be derived from the common ancestral form Cenellipsis. But a far more difficult question is the manner in which its pedigree may be constructed. The oldest family is probably the simplest, namely, Ellipsida. From this the Druppulida may be derived by production of medullary shells, the Artiscida by equatorial constriction. The Cyphinida can be produced either from the Druppulida by equatorial constriction or from the Artiscida by development of medullary shells. The Panartida appear as further developmental steps of the Cyphinida, by duplication of the chamber number; and the Zygartida as further productions of the Panartida, by increasing the number of the chambers.
The seven subfamilies of the Prunoidea can be arranged in two sections according to the presence or absence of medullary shells. The Ellipsida, Spongellipsida, and Artiscida possess a simple cortical shell, without a medullary shell; they represent the section Cenoprunida. All other families possess medullary shells, and so represent the section Coccoprunida.
Another character, which can be employed in the arrangement of the seven subfamilies in some larger groups, is the presence or absence of ring-like constrictions, by which the cortical shell is divided into chambers. I. The Monoprunida comprise all forms without any constriction, of which the Ellipsida are without a medullary shell, the Druppulida with one or two medullary shells, and the Spongurida with a spongy cortical shell. II. The Dyoprunida contain all forms with a cortical twin shell, or with two chambers separated by one equatorial constriction, of which the Artiscida are without a medullary shell and the Cyphinida have one or two medullary shells. III. The Polyprunida comprise all forms with several (three or more) constrictions, which separate four or more chambers, of which the Panartida have three constrictions and four chambers, and the Zygartida five or more constrictions and six or more chambers.
The Central Capsule of the Prunoidea is originally ellipsoidal (monaxial), and preserves this form in the greater part of the genera. In some groups, where the axis of the ellipsoid is much prolonged, it passes over to the cylindrical form (with hemispherical vaultings on both poles), as in Spongurus and Spongocore, in many Panartida and Zygartida. Very often the ellipsoidal or cylindrical capsule gets annular transverse constrictions, corresponding to those of the enveloping cortical shell (one single, equatorial stricture in the Artiscida and Cyphinida, three strictures in the Panartida, five or more in the Zygartida). In the Cenoprunida (Ellipsida and Artiscida, also in Spongellipsis) the central capsule lies freely in the cavity of the cortical shell, separated from its inner surface by the jelly-envelope; in all other groups it contains a part of the skeleton, the medullary shell and the beams which connect it with the enveloping cortical shell.
Synopsis of the Families of Prunoidea.
A. MONOPRUNIDA. Shell without transverse stricture |  | a. Shell simple, latticed (not spongy), without enclosed internal shells, | 1. Ellipsida. |
| b. Shell composed of two or more concentric latticed shells (not spongy), | 2. Druppulida. | ||
| c. Shell partially or wholly composed of an irregular spongy framework, | 3. Spongurida. | ||
B. DYOPRUNIDA. Shell bilocular, divided by an equatorial stricture into two communicating hemi-ellipsoidal shells. |  | d. Shell simple, without enclosed internal shells, | 4. Artiscida. |
| e. Shell composed of two or more concentric shells, | 5. Cyphinida. | ||
C. POLYPRUNIDA. Shell multilocular, divided by three or more parallel transverse strictures into four or more serial cameræ. |  | f. Shell with three parallel strictures and therefore four cameræ, | 6. Panartida. |
| g. Shell with five or more parallel strictures and therefore six or more cameræ, | 7. Zygartida. |
| A. MONOPRUNIDA. Shell without transverse stricture | ||||
| a. Shell simple, latticed (not spongy), without enclosed internal shells, | ||||
| 1. Ellipsida. | ||||
| b. Shell composed of two or more concentric latticed shells (not spongy), | ||||
| 2. Druppulida. | ||||
| c. Shell partially or wholly composed of an irregular spongy framework, | ||||
| 3. Spongurida. | ||||
| B. DYOPRUNIDA. Shell bilocular, divided by an equatorial stricture into two communicating hemi-ellipsoidal shells. | ||||
| d. Shell simple, without enclosed internal shells, | ||||
| 4. Artiscida. | ||||
| e. Shell composed of two or more concentric shells, | ||||
| 5. Cyphinida. | ||||
| C. POLYPRUNIDA. Shell multilocular, divided by three or more parallel transverse strictures into four or more serial cameræ. | ||||
| f. Shell with three parallel strictures and therefore four cameræ, | ||||
| 6. Panartida. | ||||
| g. Shell with five or more parallel strictures and therefore six or more cameræ, | ||||
| 7. Zygartida. | ||||