"The oyster is said to possess the most limited faculties of all shelly tribes. Immovable upon the rock or marine substance to which it is fastened, it receives no other nourishment than what the waves contribute, and indicates no other sign of life than opening and closing the valve of the shell. This genus still retains a great number of species: one section has the margin of the shell either _simple_, or _waved_, the other _folded_. _O. edulis_, common oyster, belongs to the first division. "_O. folium_ is of the second; a curious species, from the Indian and American seas: the shell is fixed to wood and to the roots of trees on the sea-shores. "_Vulsella_ is a foreign genus, from the Indian and other seas. [Sidenote: PLACUNA. ANOMIA.] "_Placuna_ does not adhere to any marine substance. The valves are flat, thin, and transparent; the very small space between them shows that the animal must be extremely flattened: there are two singular ribs at the hinge in the form of a V. "_P. placenta_, Chinese window-glass, is so transparent when young, that it serves instead of that material in China. "_Anomia._ The shells of this genus are fixed, like the oyster, to marine bodies. They live and perish on the spot where they are at first produced. I have noticed the muscle by which they attach themselves. Lamarck informs us that a hard, small operculum is to be seen at the extremity of this muscle, and fills up the _hole in the flat valve_ when the muscle is contracted. (Plate 5.) "The family _Rudista_ contains only a few genera, which will be quite uninteresting to you at present. "The next, _Brachiopoda_, has _Crania._ _Orbicula_ _Patella_. _Terebratula_ Some from _Anomia_. _Lingula_ _Patella_." Lucy could not forbear interrupting her father upon hearing the name of _Patella_. "How can that genus be mixed with the _Conchifera_?" she inquired. "The shell is _bivalve_," he replied; "raised upon a fleshy peduncle, and fixed to marine substances; the hinge is without teeth, having the form of a duck's beak; the colour a greenish tint. It is found near the Molucca isles. [Sidenote: HIPPONYX MITRATA.] "Yet more remarkable is the _Hipponyx mitrata_, a common shell, known as _Patella mitrata_, long supposed to be a univalve, the upper valve only being known. A French naturalist discovered the lower valve, and _both_ have one muscular impression in the form of a horse-shoe. "I think that it will be best to pause a little before we enter upon the study of the twelfth class, _Mollusca_, which contains most of the univalves of Linnæus." CHAPTER IX. [Sidenote: MOLLUSCA. CLEODORA.] Twelfth Class--_Mollusca._ "As I have observed that you have been very diligent in studying Lamarck since our last lesson," said Mr. Elliot, "I propose to make you acquainted with the variations in the univalve genera.