Shells of Lingula occur in the earliest Palæozoic strata, and so closely resemble those of the present day, that often no difference can be observed either in the shape of the valves or in the muscular impressions on their inner surface (Fig. 6).
Lingula occurs in the Indo-Pacific, Australia, China, Japan, and the Pacific Islands. Glottidia, a smaller form, with two small curved plates on the brachial and a ridge on the peduncle valve, is found on the American coast of the Pacific, and in the Atlantic.
Fig. 5.
Lingula anatina, removed from shell, mantle reflected, coiled arms separated slightly; a, mouth. (Marginal setæ omitted.) Ventral aspect, three-quarter face.
Fig. 16.
Lingula anatina. Interior of valves showing muscle scars.
V. Peduncle valve, D. Brachial valve.
Order 2.—Neotremata. The Discinidæ includes two genera, Discina and Discinisca (Fig. 7), with orbicular conical shells, of horny calcareous composition; both valves are conical in the former genus, but in the latter the peduncle valve is flattened. Sometimes the embryos settle down on the parent shells, and we see a mass of shells in various stages of growth, as in the specimen of Discinisca lamellosa from Peru.
Fig. 7.
Discinisca lamellosa. Peru. (After G. Sowerby.)
A group of old and young specimens; largest showing foramen in peduncle valve, the rest showing brachial valves.
