Fig. 8.
Three specimens of Crania anomala on a stone. Loch Fyne.
The Craniidæ form small limpet-like shells (Fig. 8) closely adherent to the rocks by the whole surface of the peduncle valve; although this valve is so named, no peduncle or foramen is found in this family. The Neotremata, like the Lingulidæ, are remarkable examples of persistence of type, since forms very similar to the present day Discinas and Cranias occur in the Palæozoic, Ordovician and Silurian strata.
A piece of rock, with several specimens of Crania anomala attached, is exhibited.
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.