Of the Unionidæ, or "Fresh-water Mussels," Unio, 1-3, has teeth on the hinge, and Anodon, 4, has not: it is a thin shell. The foot of these molluscs is very large, and is used in cutting the animal's way through the mud in which it burrows. Of the marine, or true Mussels, Modiola, 6-9, and Crenella, 12-17, have a portion of the shell reaching beyond the point or apex, which in Mytilus is terminal, 18-21. Dreissina, 5, is shaped like Mytilus, but lives in fresh-water docks. The Mytilidæ attach themselves by means of horny threads spun from the foot, and called a "byssus."
Shells of Arcadæ, 1-14, have the hinge characterized by a series of teeth on each side of the apex. Nucula, Arca, and Pectunculus are easily distinguished. Limopsis, 14, has a spring pit between the two rows. Our figure of Pinna, 16, is from a young specimen: it grows to great size, and spins a very silky byssus. The shell of Anomia, 18, is fixed to rocks, etc., by means of a bony button passing through a hole or sinus in the lower valve. Lima, 22-24, has a light, thin shell, and its mantle is adorned with beautiful fringes. It swims rapidly through the water by the opening and shutting of its valves, and also has the habit, in seasons of rest, of protecting itself by a network or vest of marine fragments strung together by its byssal threads.
Contains the shells of our Pectens, or Scallops, 1-16. The animals swim like Lima, as above, and also spin a byssus.
The class Brachiopoda, 17-24, so named because what seem to be organs of locomotion consist of a pair of coiled, ciliated arms, ends the list of Headless Molluscs and their bivalve shells.
Class Pteropoda, 1-4, contains minute Mollusca, with glassy shells variously formed, and wing-like expansions for swimming.
Chiton, 5-17, or "Coat of Mail," which may often be seen incrusting rocks, commences the Gasteropoda; although seemingly fixed, they are capable of locomotion, using the whole of the oval disc, which may be seen on turning them over, for the purpose. The same may be said of the "Limpet" tribe, Patella, 1-22.