The radius[3] (adopting the name used by Bruguière, Lamarck, and others) differs remarkably in appearance (though not in essence) from the walls or parietal portion, owing to the direction of the lines of growth and the state of its usually depressed surface. In the upper part the radii overlie the alæ of the adjoining compartments: in outline (r, fig. [1], [2], [3]), they are wedge-formed, with their points downwards; their summits (and this is often a useful specific character) are either parallel to the basis or as in fig. [1] and [2], oblique. The radii are sometimes not developed.
[3] The radii have been called by Ranzani and De Blainville “areæ depressæ” (the parietal portions of the compartments being the “areæ prominentes”); by Poli, “areæ interjectæ;” by Gray, “sutures;” by Coldstream, “compartments of the second order,” (the parietal portions being those of the first order); by some authors as “intersticia.” I may here add that the scuta are the “ventral valves” of Gray, the “anterior” of Ranzani, and the “inferior opercular” of De Blainville: the terga are the “posterior valves” of Gray and Ranzani, but the “superior opercular” of De Blainville: the rostrum, on the other hand, is the “anterior valve” of Ferussac and the “ventral” of De Blainville; the carina being the “dorsal valve” of the latter author.
The alæ (so called by Dr. Gray) are overlapped by the radii and by part of the walls; they usually extend only about half way down the compartment (a fig. [3], [4], [1]); their summits are either parallel to the basis or oblique. The alæ of the several compartments, together with the internal, upper, thickened surfaces of the walls, against a shoulder of which the sutural edges of the alæ abut, have been called (by Dr. Gray) the sheath (vagina). The upper and greater portion of the sheath is marked by transverse lines, caused by the exuviation of the opercular membrane, as that membrane may be called, which unites the operculum all round to the sheath, or upper internal surface of the shell.
The carina has always two alæ, as in fig. [4].
The carino-lateral and lateral compartments have always an ala on one (the rostral) side, and a radius on the other (the carinal) side, as in fig. [3].
The rostro-lateral compartment (when present) has always radii on both sides, as in fig. [2].
The rostrum has normally alæ on both sides, as in fig. [4], but very often from fusion with the rostro-lateral compartments on both sides, it has radii on both sides, as in fig. [2].
The walls of the shell, the basis, and the radii, are in very many cases composed of an outer and inner lamina, united together by longitudinal septa; a set of tubes or pores being thus formed. The points of the longitudinal septa generally project beyond the laminæ, and are denticulated on both sides (see woodcut, further on;) the septa are sometimes branched, several irregular rows of pores between the two laminæ being then formed (see Pl. [7], fig. [3 b], and Pl. [10], fig. [1 g], [1 h]).
Operculum, or opercular valves.—These consist of a pair of scuta and a pair of terga. They are joined to the sheath of the shell by the opercular membrane.