The compartment, at that end of the shell where the cirri are exserted through the aperture or lips of the operculum, is called the _carina_ (fig. 1); the compartment opposite to it is the _rostrum_,--these two lying at the ends of the longitudinal axis of the shell. Those on the sides are the _lateral compartments_; that nearest the carina, being the _carino-lateral_ (_testæ valva carino-lateralis_), that nearest the rostrum, the _rostro-lateral_, and the middle one, simply the _lateral_ compartment; but these three compartments are rarely present together. The _rostro-lateral_ compartment, which always resembles fig. 2, and may be always known by having radii on both sides, is often absent; and not rarely the lateral and carino-lateral compartments are confounded together, or one is absent; in such cases the compartment that is left is simply called the lateral one. The compartments are separated from each other by _sutures_, which are often so fine and close as to be distinguished with difficulty. The edge of a compartment, which can only be seen when disarticulated from its neighbour, I have called the sutural edge (_acies suturalis_). Each separate _compartment_ consists of a _wall_ (_paries_), or _parietal portion_ (_pp_ in woodcuts), which always grows downwards, and forms the basal margin; and is furnished on the two sides either with _alæ_ (fig. 4), or with _radii_ (fig. 2), or with an ala on one side and a radius (fig. 3) on the other. 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.