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: 1. _Tubicolæ_ (plates, fig. 44 to 54), having shelly tubes besides the valves. This family contains the genera Aspergillum, Clavagella, Teredina, Teredo, Xylophaga, Fistulana, and Gastrochæna. 2. _Pholadaria_ (plates, fig. 55 to 59), cylindrical, living in holes in rocks pierced by the animals. Lamarck places in this family the genera Pholas and Gastrochæna, the last of which belongs more properly to the family Tubicolæ, as placed above.