In Lamarck's "Histoire Naturelle des Animaux sans Vertebres," he divides the invertebrata into classes, the 9th, 10th, and 11th of which include animals possessed of shells properly so called. These are the Annelides, the Cirripedes, the Conchifera, and the Mollusca.
The class Annelides constitutes the 9th, and is divided into three orders, namely, the "Apodes," "Antennees," and "Sedentaires"; the last of which, Sedentaria, alone contains testaceous animals. This order includes tubular shells, which, with the exception of Dentalium, are irregularly twisted, and attached to each other, or to extraneous substances. The first family Dorsalia, contains the genus Siliquaria (plates, fig. 1), known from the Serpulæ, by the slit which passes through the whole length of the shell on the upper surface of the tube. The second family, Maldania, has the genus Dentalium (plates, fig. 2), a species of which are commonly known by the name of "tooth shells"; these are regularly formed, curved conical tubes, open at both extremities. The third family, Serpulacea, includes the genera Serpula, Spirorbis, Galeolaria, Vermilia, Spiroglyphus, and Magilus. The only shell that a learner would be likely to place among these incorrectly, according to the system, is the Vermetus (plates, fig. 345), which being regularly spiral at the apicial extremity, has been placed among the Mollusca; to which situation the whole of the shells under consideration have a better title than is generally supposed. It should be mentioned that the Serpulacea are provided with opercula.
Class Cirripedes.
This class constitutes the tenth of invertebrated animals, and receives its name from the jointed and ciliated branchia which protrude between the opercular valves. They are Multivalve shells, and were all included in the single genus Lepas in the system of Linnæus, and are commonly known by the name "Barnacles." Lamarck has, however, divided them into two distinct orders. First, the Sessile Cirripedes or those which being composed of several valves, joined to each other, side by side in a circle, are attached to each other, or to submarine bodies by the basal portion of their own substance, and form a hollow, irregular cone, with the aperture above closed by an operculum consisting of two or more valves. Secondly, the Pedunculated Cirripedes, which are composed of valves placed in pairs against each other, so as to form a flattened disc attached by means of a tendinous tube called a peduncle. The first of these orders includes the genera Tubicinella, Coronula, Platylepas, Clitia, Conia, Elmineus, Catophragmus, Octomeris, Balanus, Creusia, Nobia, Savignium, Pyrgoma, Adna, Megatréma. The second contains the genera Pentelasmis, Scalpellum, Smilium, Pollicipes, Bisnæus, Lithotrya, Ibla, Octolasmis, Cineras, Otion.
Conchological writers are not agreed as to the propriety of allowing the above to enter into the present science.
Class Conchifera.
The shell of a conchiferous animal is always bivalve, composed of two pieces placed opposite to each other, joined at the dorsal margins by an elastic hinge. All true bivalve shells belong to animals of this class; and the correspondence between the shell and the animal is so true that on examining an empty bivalve shell we can not only determine that its inhabitant belonged to this class, but also decide on the particular order and family in which it should be placed, without seeing the soft parts.
The first general division of Conchifera is that which results from observing the muscular impressions, or marks made on the inner surface of the valve by the insertion of the adductor muscles. All Conchifera are divided into two orders, as follows:
First Order, Conchifera Dimyaria.
Having two adductor muscles, and consequently two impressions in each valve. They are separated into the following families: