Page:Scientific results HMS Challenger vol 18 part 1.djvu/496

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	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	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.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	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.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	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.