Let it be observed that patelliform, or limpet-shaped shells are not all symmetrical; Umbrella, Siphonaria, Ancylus, &c. will form exceptions, of which we have yet to speak. And the learner may also be reminded that the Limpets themselves are not _all_ regular in their form: for as they adhere to rocks and other rough surfaces, and are so little locomotive, in many instances they partake of the inequalities of the surface, and conform to its irregularities. This adherence is not effected by any agglutinating power in the animal, nor by any tendinous process like that described above; but simply by means of the foot of the animal acting as a sucker. The next variation in symmetrical univalves is to be observed in the tubular, curved form, the example of which will be the Dentalium, fig. 12. [Illustration: Dentalium Elephantinum.] This has an opening at the anterior termination _a_, called the aperture. The opening at the posterior end (_p_) is named a fissure, or perforation. The ribs running along the sides of the shell are _longitudinal_, or radiating. And the lines round the circumference are _lines of growth_, or _concentric_--each one having in succession, at earlier stages of growth, formed the aperture. They are described as concentric, or transverse. _Symmetrical Convolute Univalves._ The Nautilus, the Spirula, the Scaphite, and the Ammonite are the leading types of this form; but when we use the term symmetrical, in reference to these, the word must not be understood in its strictest sense, for no shell is _perfectly_ symmetrical: but it means that there is no perceptible difference in the proportion of the two sides; as in the human body, the right side is larger and more powerful than the left, yet to a degree so small that it gives no apparent bias to the figure. CHAMBERED SHELLS. Many of the shells now under consideration are chambered, that is, the internal cavity is divided into separate compartments by plates reaching across it, named _Septa_; and the only connection between the chambers is formed by the small pipes passing through them, to which the name of Siphon is attached. _Septa._ The septa are _simple_ in some species, as in the Nautilus, fig. 13. In others they are _undulated_, having waved edges, as in some species of Ammonites; in others they are _angulated_, as in Goniatites, fig. 480 in the plates; and in the greater number of instances, among the Ammonites, they are _arborescent_, or branched. [Illustration: 13. Section of Nautilus.--14. Undulating Septa.--15. Arborescent Septa.] In the above section of a Nautilus, fig. 13, diminished in size, showing the whorls and chambers (_c_), it will be seen that the edges of the septa (_s_) are formed in one simple curve. In fig. 14, the upper part of an Ammonite, the undulating line will be seen; and in fig. 15 a specimen is given of the arborescent septa. _Siphon._