_Varices_ are transverse ribs which cross the whorls of shells in some species of Buccinum, Murex, and Tritonia, and exemplified in the Cassidaria. Varices are formed by the periodical growth of the shells, these being the margin of the outer lip, to which the animal has attached its periodical enlargements. In some species they have more the form of sutures than ribs; this is owing to the margin of the outer lip being but slightly developed. _Ribs_ are those longitudinal and transverse protuberances which are in many of the univalve shells. Plate III. fig. 12, _R R R R_. _Teeth_ of univalves, or tooth-shaped protuberances, are fine, white laminæ, or ridges, running spirally backwards, in a parallel direction to each other; those on the exterior lip may, in most instances, be traced through the outside of the shell, and are nearly alike in length. Plate III. fig. 13, _a_. _Epidermis_ is a skin, or cuticle, covering the exterior surface of shells, destined by nature to protect their surface from being injured. It is membranaceous, somewhat similar to the periosteum which covers the bones of animals. This substance is the production of the animal inhabiting the shell; it is uniformly observed in some species, and not at all in others. Shells with a rugged or uneven surface have almost always this epidermis. In some it is strong, laminated, velvety, fibrous, or rough, often beset with long hairs, and in others very thin, smooth, and pellucid, and admits the colours of the shell to shine through it. In some species it is so dusky that it entirely obscures the beautiful colouring of the shell beneath. Although many shells are very beautiful, even with this cuticle on them, they are much more so when it is removed; but I would by no means advise the collector of shells to remove it, unless he may have several duplicates of the same species; and then he may do it by way of variety. It is always preferable, however, to keep them in a state of nature. A shell with epidermis is represented in Plate III. fig. _X_; and the effect of the epidermis removed. All other protuberances, furrows, &c., will be described at the end of this work in a Glossary of Terms used in the Science. DESCRIPTION OF PLATE IV. HINGES OF BIVALVE SHELLS. Fig. 1. Hinge of the Unio pictorum, _a_ Primary tooth, _c c_ double teeth. Fig. 2. Hinge of the _Solen_. _b_ Lateral teeth. Fig. 3. Hinge of the Lucina radula, _a a_ Primary teeth, _n_ cartilage. Fig. 4. Hinge of the _Cardium_. _b_ Lateral tooth, _d d_ middle teeth, _l l_ spines. Fig. 5. Hinge of the Lutraria. Fig. 6. Inside of both valves of the _Donax trunculus_. _A_ is the left valve, and _B_ is the right valve, _b_ lateral teeth, _e_ primary complicated tooth, or cleft in the middle, _m m_ crenulated margin. Fig. 7. Hinge of the _Venus_, _a a_ Primary teeth. Fig. 8. Hinge of _Spondylus gæderopus_, _f f_ incurved teeth. Fig. 9. Hinge of Isocardia Cor.