Section II.—Articulata.
Order 3. Protremata.—This group, formerly very abundant, is now almost extinct, the Family Thecidiidæ representing the Order at the present day. Thecidium mediterraneum (Fig. 9) forms little oval boxes about a third of an inch in length, shaped somewhat like a pear cut in half (peduncle valve), and with a semicircular lid (brachial valve) working on a hinge on the upper flat surface. The foramen and peduncle are absent; but between the pointed end of the peduncle valve and the hinge is an area filled in by a calcareous plate characteristic of the Protremata.
Fig. 9.
Thecidium mediterraneum. A, natural size. B, section through shell. Magnified.
The brachial valve opens like the lid of a snuff-box, and shuts down on the least alarm with the rapidity of lightning. The peduncle valve is fixed on the rocks by its convex surface. The species is common in the Mediterranean in from 30 to 300 fathoms, and is also found in the West Indies.
Order 4. Telotremata.—This group, which at the present day contains the largest number of species, includes the Lamp shells, so called from their resemblance to an ancient lamp. The valves are joined by a well-marked hinge, the peduncle passes through the peduncle valve through a foramen completed by two plates secreted by the mantle edges, and the brachial valve has attached to it a calcareous scaffolding of processes or loops for the support of the “arms.”
The shells in this group are frequently ridged. Their colour is usually white, but sometimes red or yellow; deep-sea forms are generally vitreous.
Rhynchonella psittacea has a black shell with a pointed incurved beak; each of the arms forms a many coiled spiral and can be protruded beyond the shell; the brachial skeleton is comparatively small and simple, consisting of two separate processes.
In Terebratulina the brachial skeleton forms a simple loop; in Magellania the loop is reflected on itself (Fig. 2).
The beautiful and unique specimen of Dyscolia wyvillii, from 390 fathoms W. Indies, is remarkable for its size, being over two inches in length. The small vitreous specimens of Terebratula wyvillii were obtained off Chili from a depth of 2160 fathoms; specimens of the same species were obtained also from a depth of 2900 fathoms in the North Pacific.