The sexes are usually separate. The reproductive cells are formed in the body-cavity. The embryo swims freely for a short time before settling down and becoming fixed. The specimens exhibited in the case are arranged according to the following classification:— Section I. INARTICULATA. │Order 1. Atremata. │Fam. _Lingulidæ_. ─────────────────────────┼──────────────────────┼────────────────────── „ │Order 2. Neotremata. │Fam. _Discinidæ_. „ │ „ │Fam. _Craniidæ_. Section II. ARTICULATA. │Order 3. Protremata. │Fam. _Thecidiidæ_. ─────────────────────────┼──────────────────────┼────────────────────── „ │Order 4. Telotremata. │Fam. _Rhynchonellidæ_. „ │ „ │Fam. _Terebratulidæ_. „ │ „ │Fam. _Terebratellidæ_. Section I.—INARTICULATA. Order 1.—ATREMATA. Family _Lingulidæ_.—The Lingulas possess emerald green or golden brown duck-bill-shaped shells. Having no hinge, the dead valves of dried shells easily fall apart. The peduncle, which is sometimes over six inches in length, passes between the pointed posterior borders of the valves. Dr. François gives a very interesting account of the habits of _Lingula anatina_ which he found living in the sand at Noumea, New Hebrides. The sole evidence of the animal’s existence is the presence, on the surface of the sand or mud, of a small, three-lobed slit (Fig. 4, upper figure). The tube (Fig. 4) in which the _Lingula_ lives is about four inches deep, flat in the upper half, rounded below. The walls of the upper flat portion simply consist of the sand with a surface coating of mucous secretion; but in the lower end the sand grains are agglutinated so as to form a distinct tube. [Illustration: Fig. 4. _Lingula anatina_ in tubes in the sand; upper figure shows trilobed opening on surface of sand. Dotted line in lower figure indicates position in retraction. (After François.) ] The edges of the mantle-folds are provided with setæ (bristles), which form three funnels protruding through the three lobes of the slit-like mouth of the sand-tube; currents enter by the lateral funnels and leave by the central. On the least alarm the animal is rapidly withdrawn as far as the centre of the tube (see the dotted line of the shell in the figure), the surface slit and upper part of the tube being obliterated. Each of the arms forms a spiral with several coils (Fig. 5). The _Lingulidæ_ are of exceptional interest, in that they furnish a very remarkable example of “persistence of type.” 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).