Cuttlefish have a unique internal shell, the cuttlebone. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. The chalky plate, or cuttlebone, of cuttlefish also acts as armour. The cuttlebone lacks a pronounced spine; if present, it is small and chitinous. The whole margin of the cuttlebone is surrounded by chitin. Characteristic of the genus "Metasepia", the cuttlebone is rhomboidal in outline. The dorsal surface of the cuttlebone is yellowish and evenly convex. The two members of this genus are characterised by a small, thick, diamond-shaped cuttlebone. The cuttlebone is relatively ellipsoid in shape. According to studies, high CO concentrations, cuttlefishes tend to lay down a denser cuttlebone. A model is pushed into the cuttlebone and removed, leaving an impression. Both the anterior and posterior of the cuttlebone taper gradually to an acute point. The cuttlebone is long and thinly oblong in shape with a long posterior spine. Cuttlebone has been used since antiquity to make casts for metal. Each species' cuttlebone has a distinct shape, size, and pattern of ridges or texture.