The great sub-kingdom of the Mollusca, including the bivalve and univalve shell-fishes, makes its first appearance in the Cambrian, where its earliest representatives belong to a group, the Arm-bearers or Lamp shells (Brachiopods), held by some to be allied to worms as much as to mollusks. The oldest of all these shells are allies of the modern _Lingulæ_ (Fig. 54), some of the earliest of which are shown in Fig. 55. The modern _Lingula_ is protected by a delicate two-valved shell, composed, unlike that of most other mollusks, of phosphate of lime or bone earth. It lives on sand-banks, attached by its long flexible stalk, which it buries like a root in the bottom. Its food consists of microscopic organisms, drifted to its mouth by cilia placed on two arm-like processes, from which the group derives its name. In the modern world about one hundred species of Brachiopods are known, belonging to about twenty genera, some of which differ considerably from the Lingulæ. The genus _Terebratula_, represented at Fig. 56, is one of the most common modern as well as fossil forms, and has the valves unequal, with a round opening in one of them for the stalk, which is attached to some hard object, and there is an internal shelly loop for supporting the arms. [Illustration: FIG. 52.—_Extracrinus Briareus_. Reduced. Jurassic.] [Illustration: FIG. 53.—_Pentacrinus caput-medusæ_. Reduced. Modern.] [Illustration: FIG. 54.—_Lingula anatina_. With flexible muscular stalk. Modern.] [Illustration: FIG. 55.—Cambrian and Silurian Lingulæ. _a_, _Lingulella Matthewi_ (Hartt). Acadian group. _b_, _Lingula quadrata_ (Hall). Siluro-Cambrian. _c_, _Lingulella prima_ (Hall). Potsdam. _d_, _Lingulella antiqua_ (Hall). Potsdam.]